mon/OSDMonitor: enforce caps when creating/deleting unmanaged snapshots

mon/OSDMonitor: enforce caps when creating/deleting unmanaged snapshots

The entity will require write access to the OSD service or permission
for the synthetic "osd pool op unmanaged-snap" command.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 4972e054b32c8200600f27564d50d443e683153e)

Conflicts:
	src/mon/OSDMonitor.cc: use 'bufferlist::begin' instead of 'bufferlist::cbegin'
(cherry picked from commit 3dfb23d036c068ec5166151ed63628738d68908a)

Conflicts:
	src/mon/CMakeLists.txt: added 'OSDCap.cc'