NSX: Add ability to retry on 503's returned by the controller

There are a number of circumstances where the NSX controller
may return 503. Currently the API client does not retry, so
this patch adds a retry logic with timeout.

Closes-bug: #1284277

Change-Id: I85df087d5ae409e6cb5c35eb171e89346abe81f4
(cherry picked from commit cb99c08e968685c6f59bab2e49f86d56a2ec509d)
Merge "NSX: Add ability to retry on 503's returned by the controller" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable