mon: Monitor: MMonGetMap doesn't require caps

We are dropping the requirement for MON_CAP_R for MMonGetMap.

Reason is simple enough: clients may need to contact the monitors and
obtain the latest monmap before authenticating.  This happens, for
instance, when a client calls MonClient::get_monmap_privately().  The
osd uses this function during mkfs, prior to initializing a keyring or
even so much as existing.

Fixes: #9859

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>
Merge pull request #2777 from ceph/wip-9859

mon: Monitor: MMonGetMap doesn't require caps

Reviewed-by: Sage Weil <sage@redhat.com>