mgr/smb: move logic validating cluster changes to handler

mgr/smb: move logic validating cluster changes to handler

When I first wrote the internal module for working with serialized
objects in the internal store it seems to make sense to do some
validation in the cluster class. At the time it was one of the few
places in the code that would have both the new and old cluster objects
for comparison. However, the code evolved and the handler module grew
more responsibility for validation especially as it needed to do
cross-object validation (do the cluster and share values align, etc).
In addition, I initially couldn't find the code that handled the
validation because I forgot there was validation in internal.py.

Move the logic used to validate that certain properties of a cluster are
unchanged into handler.py and out of internal.py. This makes the classes
in internal.py even more regular as a bonus.

Signed-off-by: John Mulligan <jmulligan@redhat.com>