Run "glance-manage db_sync" on SUSE when HA is enabled

So far, we were not running it because the init script on SUSE does run
it. But with HA enabled, the service starts at the same time on all
nodes, which may result in failures on some nodes because db_sync can't
be run multiple times simultaneously. The end result is that the
pacemaker resource was possilby failing on some nodes, which then
required some manual cleanup.
Only run the db_sync the very first time (on SUSE, at least)
Do the db sync only on initial run, and on all platforms
Merge pull request #204 from vuntz/db_sync-for-ha

Run "glance-manage db_sync" on SUSE when HA is enabled