Bug:1253 - Fix NPE in config pusher

Add better error message when ConfigSnapshotHolder contract is voilated.
Fix
org.opendaylight.controller.config.persist.storage.file.xml.model.ConfigSnapshot
to return empty set instead of null in getCapabilities()

Change-Id: I9783cccbe53ae69e322e1d5660ad1f3b52b007fa
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>
Merge "Bug:1253 - Fix NPE in config pusher"