Create Physical Port By Listening on DS changes

Create Physical Port By Listening on DS changes

PatchSet 2&3:
share code and change it according comments

PatchSet 4:
rebase on 'Create Physical Switch'

PathchSet 5:
1.rebase on 'Yang changes and cleanup'
2.finish creating physical port
3.rewrite HwvtepOperationalState

PatchSet 6:
change code according comments

PatchSet 7:
rebase on master branch

PathchSet 8:
fix a nullpoint bug

Change-Id: I425336aea3b055ee4d7e75bd26490e194340e3af
Signed-off-by: Peng Zhang <pzhang@ctbri.com.cn>