Fix 'INPORT' keyword in port field

Fix 'INPORT' keyword in port field

- also fix NPE in serialization by using wrong port number

Enumeration from opeflow java yang files is correct used keyword 'IN_PORT'.
Openflowplugin with the wrong translation (getString instead of getName) used the
keyword 'INPORT'. This fix use the correct name from enumeration but allow use
the legacy 'INPORT' keyword for next release.
Also prevent to throw NPE by using wrong keyword in port field.

Change-Id: If8b3d301d73169fd158a546a233928ab0c65d1c3
Fix: Bug 2095
Signed-off-by: Jozef Bacigal <jozef.bacigal@pantheon.tech>