Extend clustered-app-config to read default data from XML file

The default data can be specified in the clustered-app-config element
but it's also useful for scripting/automation or convenience to be able
to specify the default data in external XML file. The
clustered-app-config will now look for a file of the form

  <yang module name>_<container name>.xml

in a well-known location, etc/opendaylight/datastore/initial/config.

The XML file name can also be explicitly specified via the
"default-config-file-name" attribute.

Change-Id: Id310ef5ae121b8b9444a2102b93c3e382e421687
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>