Use new synchronization helpers to avoid races when deploying with HA

There are a number of possible races when deploying ceilometer for the
first time:

 - when creating of the database / database user
 - when registering the endpoint and creating users in keystone
 - running the db sync
Wait for mongodb on all nodes of the cluster, not just the founder
Merge pull request #102 from vuntz/sync-marks

Use new synchronization helpers to avoid races when deploying with HA