mon/MDSMonitor: fix msg on idempotent `fs rm`

Was outputting trailing "unrecognised command"
because we returned 0 instead of setting r=0.

Signed-off-by: John Spray <john.spray@redhat.com>
mon: carry last_failure_osd_epoch across `fs new`

So that new MDSs in a new filesystem are guaranteed
to be up to date with anything we blacklisted
from a filesystem coming before.

Signed-off-by: John Spray <john.spray@redhat.com>
qa: generalise cephtool for vstart+MDS

Previously this test assumed no pre-existing
filesystem and no MDS running.  Generalize it
to nuke any existing filesystems found before
running, so that you can use it inside a vstart
cluster that had MDS>0.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2119 from ceph/wip-vstart-existing-mds

Wip vstart existing mds

Reviewed-by: Sage Weil <sage@redhat.com>