Add configurable ovsdb monitor respawn interval

If minimize_polling=True and the ovsdb-client process died, the
default respawn interval of 0 would mean that we'd start polling
again. This patch adds the option ovsdb_monitor_respawn_interval and
defaults it to 30 seconds.

Change-Id: I12c1e05ec4d6d2bd4d84024a91116e2ac3974868
Close-Bug: 1243867
(cherry picked from commit 14950e492ad4423aa6f8719f7178b0477294ba5a)
Merge "Add configurable ovsdb monitor respawn interval" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable