mgr/cephadm: do not worry about even # of monitors

mgr/cephadm: do not worry about even # of monitors

Ceph works just fine with an even number of monitors.

Upside: more copies of critical cluster data
Downside: we can tolerate the same number of down mons as N-1, and now
 we are slightly more likely to have a failing mon because we have 1 more
 that might fail.

On balance it's not clear that have one fewer mon is any better, so avoid
the confusion and complexity of second-guessing ourselves.

Signed-off-by: Sage Weil <sage@newdream.net>
(cherry picked from commit ba88a8e5bf81431c37011ba96ffc843eb970683f)