migrate-ds: fix compat plugin check
After ACI refactoring, admin cannot read Schema Compatibility plugin configuration and therefore migrade-ds won't find if compat plugin is enabled.
Now the check si done by looking if cn=compat subtree is present.
https://fedorahosted.org/freeipa/ticket/4825
Reviewed-By: Jan Cholasta <jcholast@redhat.com>