mgr/smb: add a cluster resource field to manage clustering

mgr/smb: add a cluster resource field to manage clustering

Add a new `clustering` field to the smb cluster resource. This field can
be used to select either automatic clustering with ctdb, or disable it,
or require it. The default is automatic and is based on the count value
in the placement spec. A count of 1 disables clustering and any other
value it is enabled.

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