Bug 2412: Expose Mountpoints on proper path

Lists such as topology, node were not doubled
in the YANG Instance Identifier used as a key for mountpoints,
which prevented Binding compontents from accessing
them.

This patch fixes also bug in restconf, which was
also producing incorrect instance identifiers.

Change-Id: I3122163d00d4c23fcccc4bbffdf3d0113744c98d
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>