BSN: Optimistic locking strategy for consistency

Add an optimistic locking strategy for the Big Switch
server manager so multiple Neutron servers wanting to
communicate with the backend do not receive the consistency
hash for use simultaneously.

Conflicts:

	neutron/plugins/bigswitch/servermanager.py

Closes-Bug: #1374261
Change-Id: Ifa5a7c9749952bc2785a9bf3fed69ad55bf21acc
(cherry picked from commit 2b2d6821015d51910b7318e231df90f6298a94e7)