grid: Honour PROMISE_TIMEOUT instead of hardcoded value

Despite PROMISE_TIMEOUT is configurable on module level, the hardcoded value
took precedence. In order to honour it, just pass None while instantiating
the class.

slapos/grid/slapgrid.py

@@ -323,7 +323,7 @@ class Slapgrid(object):
                upload_dir_url=None,
                master_ca_file=None,
                certificate_repository_path=None,
-               promise_timeout=3,
+               promise_timeout=None,
                shacache_ca_file=None,
                shacache_cert_file=None,
                shacache_key_file=None,