Update the description of the role driver option
Update the description of the role driver option

The ldap driver for roles is no longer supported
since mitaka.

Change-Id: Ic9dbc2c97522b52d6044388cafb90226452735df

rajalokan/keystone,openstack/keystone,cernops/keystone,openstack/keystone,openstack/keystone,cernops/keystone,mahak/keystone,klmitch/keystone,ilay09/keystone,ilay09/keystone,mahak/keystone,klmitch/keystone,mahak/keystone,rajalokan/keystone,rajalokan/keystone,ilay09/keystone