ceph_ctl: remove 'only three hosts' test from ExternalCephCtl

Signed-off-by: Dan Mick <dan.mick@redhat.com>
tests/test_monitoring: skip test_mon_down if < 3 mons

There will never be any update from a cluster with < 3 mons when
you take one down.

Signed-off-by: Dan Mick <dan.mick@redhat.com>
tests: change 2.7-only constructs to 2.6-compatible ones

Signed-off-by: Dan Mick <dan.mick@redhat.com>
tests/test_pool_management: avoid mutable default arg

Signed-off-by: Dan Mick <dan.mick@redhat.com>