Bug 6325 - upgrade draft11 to draft15 - change media types

  * remove media types(for +json and +xml in all cases):
    * yang.api
    * yang.datastore
    * yang.data
    * yang.operation
    * yang.errors
  * add new media type:
    * for xml - yang-data
    * for json - yang-data+json

Change-Id: I9c1e3a28872c47f691d342a4982eaa3f8bbae16c
Signed-off-by: Jakub Toth <jatoth@cisco.com>