tests/cluster: speedup waiting a bit

NEO functional tests use pdb.wait() in a few places, for example in
NEOCluster .run(), .start() and .expectCondition(). The wait
implementation uses polling with exponentially growing wait period.

With the following instrumentation

	--- a/neo/tests/cluster.py
	+++ b/neo/tests/cluster.py
	@@ -236,6 +236,7 @@ def wait(self, test, timeout):
	                         return False
	             finally:
	                 cluster_dict.release()
	+            print 'next_sleep:', next_sleep
	             sleep(next_sleep)
	         return True

during execution of functional tests it is not uncommon to see the
following sleep periods

	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.15
	next_sleep: 0.225
	next_sleep: 0.3375
	next_sleep: 0.50625
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.15
	next_sleep: 0.225
	next_sleep: 0.3375
	next_sleep: 0.50625
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.15
	next_sleep: 0.225
	next_sleep: 0.3375
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.1
	next_sleep: 0.15
	next_sleep: 0.225
	next_sleep: 0.3375
	next_sleep: 0.50625

.

Without going into reworking the wait mechanism to use real
notifications instead of polling, it was observed that the exponential
progression tends to create too coarse sleeps. Initial 0.1s interval was
found to be also too much.

This patch remove the exponential period growth and reduces period by order
of one magnitude. For functional tests timings on my computer it is thus:

before patch:

	Functional tests

	28 Tests, 0 Failed

	Title                     : TestRunner
	Date                      : 2018-04-04
	Node                      : deco
	Machine                   : x86_64
	System                    : Linux
	Python                    : 2.7.14

	Directory                 : /tmp/neo_tests/1522868674.115798
	Status                    : 100.000%
	NEO_TESTS_ADAPTER         : SQLite

	                               NEO TESTS REPORT

	              Test Module |  run  | unexpected | expected | skipped |  time
	--------------------------+-------+------------+----------+---------+----------
	                   Client |    6  |       .    |      .   |     .   |   8.51s
	                  Cluster |    7  |       .    |      .   |     .   |   9.84s
	                   Master |    4  |       .    |      .   |     .   |   9.68s
	                  Storage |   11  |       .    |      .   |     .   |  20.76s
	--------------------------+-------+------------+----------+---------+----------
	     neo.tests.functional |       |            |          |         |
	--------------------------+-------+------------+----------+---------+----------
	                  Summary |   28  |       .    |      .   |     .   |  48.79s
	--------------------------+-------+------------+----------+---------+----------

after patch:

	Functional tests

	28 Tests, 0 Failed

	Title                     : TestRunner
	Date                      : 2018-04-04
	Node                      : deco
	Machine                   : x86_64
	System                    : Linux
	Python                    : 2.7.14

	Directory                 : /tmp/neo_tests/1522868527.624376
	Status                    : 100.000%
	NEO_TESTS_ADAPTER         : SQLite

	                               NEO TESTS REPORT

	              Test Module |  run  | unexpected | expected | skipped |  time
	--------------------------+-------+------------+----------+---------+----------
	                   Client |    6  |       .    |      .   |     .   |   7.38s
	                  Cluster |    7  |       .    |      .   |     .   |   7.05s
	                   Master |    4  |       .    |      .   |     .   |   8.22s
	                  Storage |   11  |       .    |      .   |     .   |  19.22s
	--------------------------+-------+------------+----------+---------+----------
	     neo.tests.functional |       |            |          |         |
	--------------------------+-------+------------+----------+---------+----------
	                  Summary |   28  |       .    |      .   |     .   |  41.87s
	--------------------------+-------+------------+----------+---------+----------

in other words ~ 10% improvement for the whole time to run functional tests.

/reviewed-by @vpelletier, @jm
/reviewed-on nexedi/neoppod!10

neo/tests/cluster.py

@@ -221,7 +221,7 @@ class ClusterPdb(object):
 
     def wait(self, test, timeout):
         end_time = time() + timeout
-        period = 0.1
+        period = 0.01
         while not test():
             cluster_dict.acquire()
             try:
@@ -232,7 +232,6 @@ class ClusterPdb(object):
                     next_sleep = max(last_pdb + timeout, end_time) - time()
                     if next_sleep > period:
                         next_sleep = period
-                        period *= 1.5
                     elif next_sleep < 0:
                         return False
             finally: