Add the manila-data service into manila guide

the manila-data is missing [0][1]

[0]: https://docs.openstack.org/manila/latest/install/get-started-with-shared-file-systems.html
[1]: https://github.com/openstack/kolla-ansible/blob/master/ansible/roles/manila/defaults/main.yml#L34

Change-Id: I6936d533700637a4f4a160f776b73efc75c4b891

doc/source/reference/manila-guide.rst

@@ -9,6 +9,7 @@ Overview
 Currently, Kolla can deploy following manila services:
 
 * manila-api
+* manila-data
 * manila-scheduler
 * manila-share