mon: disallow post-hammer OSDs if there are up pre-hammer OSDs

mon: disallow post-hammer OSDs if there are up pre-hammer OSDs

Force *all* OSDs to upgrade to hammer before allowing post-hammer
OSDs to join.  This prevents any pre-hammer OSDs from running at
the same time as a post-hammer OSD.

This commit, as well as the definition of the sentinal post-hammer
feature, should get backported to hammer stable series.

Backport: hammer
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit e38d60e94503c7e8511a60a8dceceb3cd39c2bd8)

# Conflicts:
#	src/mon/OSDMonitor.cc