Bug 5509 - HTTP Patch in Restconf doesn't support general absolute or relative target...

Bug 5509 - HTTP Patch in Restconf doesn't support general absolute or relative target xpath

- yang patch for xml media now supports relative or absolute xpaths
- only create, merge, replace and insert operations require value to be present
- if value is not present with create, merge, replace or insert operation
then error 400 is returned
- if value is used with other operations then error 400 is returned
- deleting sublist values instead of full parent
- support for multiple values
- added unit tests

Change-Id: I9828ceb7368760642b22b3a336884b949f0edfcc
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>