Bug 6746 - Restconf: Not working GET operation on mount points

Bug 6746 - Restconf: Not working GET operation on mount points

- support for reading data on mount points
- improved merge for config and state data:
when identifiers for data in config and state are the same
then merge must by done at one level down, otherwise state
overrides config or vice versa
- identiers are parsed to identifiers context with reference
to mount points when data are behind mount point
- adapted and added unit tests

Change-Id: I195b3195c0eaa47effd7759e60611a3f420c9a6b
Signed-off-by: Jakub Toth <jatoth@cisco.com>