mgr/volumes: create extension to deploy and configure MDSs in response to degraded file system
The volumes plugin should no longer create an MDS for each volume. Instead, a MDS deployment service should regularly evaluate the state of the FSMap and look for any file system that requires more standbys. The service should then deploy MDSs until the file systems are all healthy. Additionally, if there are too many standbys, stop the extra MDS daemons.