mgr/cephadm: add get_dependencies to smb service class

mgr/cephadm: add get_dependencies to smb service class

Add a new get_dependencies method to the smb service class.
For now, the function returns one special dependency derived
from a hash digest of the external ceph cluster parameters,
to cause the service to be reloaded when these params change.
This approach is borrowed from a similar behavior of the nfs
service.

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