mon: make tiebreaker mon optional in stretch-mode

mon: make tiebreaker mon optional in stretch-mode

Motivation:
To future support EC stretch feature, we need to
simplify how we enable stretch-mode

Solution:
Make tiebreaker argument optional.

old:
```
ceph mon enable_stretch_mode <tiebreaker_mon> <new_crush_rule>
<dividing_bucket>
```
new:
ceph mon enable_stretch_mode <tiebreaker_mon (optional)> <new_crush_rule>
<dividing_bucket>

Ceph will try to select a tiebreaker mon that resides in
the crush <dividing_bucket> type but doesn't belong
to any of the data sites which the OSDs resides in.

Also created a helper function
`MonmapMonitor::validate_and_enable_stretch_mode`
inside `MonmapMonitor::try_enable_stretch_mode`
making the logic unittestable

Moreover, ceph mon enable_stretch_mode will
automatically set monitor election strategy to Connectivity.

We now also enforce that at least 1 monitor exists for each data zone.

Signed-off-by: Kamoltat (Junior) Sirivadhna <ksirivad@redhat.com>