Getter for port-capability in port-mapping 2.2.1

- There are two ways the 2.2.1 device can advertise
  supported port-capabilities
  a. Using the leaf list supported-interface-capability
     under ports list
  b. Using port-capabilities tree.
- Currently port-mapping only supports case (a).
- Add code to provide support for case (b).

JIRA: TRNSPRTPCE-610
Change-Id: I2e6900ccafa3e50d44d59d55c97acf824c28639e
Signed-off-by: Balagangadhar Bathula <bb4341@att.com>
(cherry picked from commit 6758b3817a44e31150a6ebea828a94d46241a63d)