validate if the router has external gateway interface set

If the router wants to work with vpn service, we must
make sure external gateway interface is set.

This patch cannot prevent user from clearing the gateway interface
of the router after the vpnservice is created.

Change-Id: If0f00def949b31c1e3da7a2cd055454567201e4c
Closes-Bug: #1258379
Enable hacking H233 rule

H233: Check that all occurrences look like print functions, not print
operator.

Change-Id: I4600eb32e1a8ecba13fc252d7ef08681672a40a7
improve UT coverage for nicira_db operations

Also, ensure that a rollback is issued in the
event of any DB error.

Change-Id: I706db09f11fb5f6b369a1bd0cb1016df7d0359b5
Related-bug: #1265472
Merge "improve UT coverage for nicira_db operations"
Merge "validate if the router has external gateway interface set"
Merge "Enable hacking H233 rule"