Add again mdsal-singleton but remove prefix DOM
We have only one implementation of ClusterSingletonServiceProvider
(DOM implementation) and we'd like to present it without DOM prefix.
* remove DOM prefix from ConfigSubsystem yang
* again add the odl-mdsal-singleton-dom feature
* again add CSSProvider in 06-clustered-entity-ownership.xml file
depends on: https://git.opendaylight.org/gerrit/#/c/42294/
Change-Id: Ieae0d462fe9fa523b2b1b18528759e0614b0225f
Signed-off-by: Vaclav Demcak <vdemcak@cisco.com>