Big Switch: Retry on 503 errors from backend

Retries requests to the backend controller up to 3
additional times with 3 seconds in between each request
if a 503 service unavailable message was returned.
The scenarios that return 503 messages from floodlight
are normally short lived locks for things like OpenStack
synchronization or upgrade blocks. Retrying should work
in the majority of cases.

Conflicts:
	neutron/tests/unit/bigswitch/test_servermanager.py

Closes-Bug: #1357105
Change-Id: Ifacd3a384cfc797ba6d6af5f3c8649c333473259
(cherry picked from commit 48a2221648f490540fdf1ee099d39b8e5230f053)
Merge "Big Switch: Retry on 503 errors from backend" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack/quantum into stable/icehouse