Fix rbd showmapped output parsing

'rbd showmapped' output formatting differs between older and newer versions of
the ceph tools. Try to use json output formatting if available (currently
available only in the ceph master branch). For bobtail, argonaut and older
releases fallback to manually parsing the 'rbd showmapped' output, handling the
differences in the output format for each rbd version correctly.

Signed-off-by: Stratos Psomadakis <psomas@grnet.gr>
Reviewed-by: Guido Trotter <ultrotter@google.com>