mon/OSDMonitor: require mon_allow_pool_delete = true to remove pools

mon/OSDMonitor: require mon_allow_pool_delete = true to remove pools

This is a simple safety check.  Since we default to true it is currently
opt-in.

Backport: giant, firefly
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 6b982e4cc00f9f201d7fbffa0282f8f3295f2309)

Conflicts:
     src/test/mon/osd-pool-create.sh

     Changed $CEPH_MON to 127.0.0.1 -- the CEPH_MON was introduced after
     firefly to allow tests to run in parallel. Back in firefly all tests
     use the same port because 127.0.0.1 was hardcoded. We can't
     conveniently backport all that's necessary for tests to run in
     parallel, therefore we keep the 127.0.0.1 hardcoded.

Conflicts:
	src/test/mon/osd-pool-create.sh
            TEST_no_pool_delete() follows a different test than in master