Fix nova-compute fails to start if quantum is down

When nova-compute starts it needs to check the state of instances that
were previously running and needs to query quantum. Previously, if
it queried quantum and quantum raised an error or was unreachable
nova-compute would terminate. This patch adds a retry loop to prevent
this from happening.

Fixes bug 1186357

Change-Id: I0d865e464f7bf60a0b6dc4ca8a42436d16e29790
Merge "Fix nova-compute fails to start if quantum is down"