OVS lib defer apply doesn't handle concurrency

The OVS lib deferred apply methods use a dict to save flows to add,
modify or delete when deffered apply is switched off.
If another thread adds, modifies or deletes flows on that dict during
another process called deffered_apply_off, its flows could be ignored.

This fix stash reference flows list and point the flows list to a new
cleared flows list. Then, it applies flows from the stashed flows list.

Closes-bug: #1263866
(cherry picked from commit 501213686886baccd3280e10b8856a25d3517519)

Conflicts:
	neutron/tests/unit/agent/linux/test_ovs_lib.py

Change-Id: I3b22c7ec055934d7f30067924c7a7fd71b2f6081
Merge "OVS lib defer apply doesn't handle concurrency" into stable/havana