mon/MDSMonitor: make legacy 'newfs' command idempotent

We need to return success if we get a dup command.  Simply check whether
the fs is already enabled with the same pools and name.

Fixes: #8857
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2113 from ceph/wip-8857

mon/MDSMonitor: make legacy 'newfs' command idempotent

Reviewed-by: John Spray <john.spray@redhat.com>
qa/workunits/cephtool/test.sh: fix erasure_code_profile get test

I broke this in ce9f12d7a2202948532fed9da4d763ed03f6b8fa (the pool isn't
type erasure).

Signed-off-by: Sage Weil <sage@redhat.com>
doc: Add Note about European mirror in Quick Start
AUTHORS

Signed-off-by: Sage Weil <sage@inktank.com>