Move mock pinning into stack/slapos.cfg

Several places pin mock egg individually. There are pins to `mock = 3.0.5`
and pins to `mock = 2.0.0:whl`. Mock 3.0.5 is the last version that
supports Python2 and can be considered as "current" in our context today.

For nxdtest we are going to add pytest-mock, which depends on the mock,
which, if not consolidated, will need to add another `mock = 3.0.5`
somewhere.

-> Stop doing that and move mock pinning into stack/slapos.cfg.
This way it will be even easier for upcoming Python3 support, because in
stack/slapos.cfg we can add per-Python3 [versions] with pins adjusted as
needed for py3, but all still in one single place.

Leave `mock = 2.0.0:whl` pins intact to be updated when needed by
maintainers of corresponding software releases:

    software/slapos-sr-testing/software.cfg:mock = 2.0.0:whl
    software/slapos-testing/software.cfg:mock = 2.0.0:whl
    software/wendelin/software-kerastensorflow.cfg:mock = 2.0.0:whl

component/ZODB/test-zodb4-wc2.cfg

@@ -109,7 +109,6 @@ template = inline:
 
 
 [versions]
-mock = 3.0.5
 random2 = 1.0.1
 zope.testing = 4.7
 zope.testrunner = 5.2

component/zodbtools/test-common.cfg

@@ -42,5 +42,4 @@ template = inline:
 
 
 [versions]
-mock = 3.0.5
 freezegun = 1.0.0

software/neoppod/software-common.cfg

@@ -125,7 +125,6 @@ template =
 
 [versions]
 coverage = 4.5.1
-mock = 3.0.5
 ecdsa = 0.13
 mysqlclient = 1.3.12
 pycrypto = 2.6.1

stack/slapos.cfg

@@ -169,6 +169,7 @@ itsdangerous = 0.24
 lock-file = 2.0
 lxml = 4.6.3
 meld3 = 1.0.2
+mock = 3.0.5
 more-itertools = 5.0.0
 netaddr = 0.7.19
 pathlib2 = 2.3.5