NSX: lower the severity of messages about VIF's on external networks

Set the message to INFO as there is no error to recover from or
something bad about to happen. The only place where this occurs
is during the setup of probe interfaces in DevStack.

Change-Id: I2310af00676e508646783ce769a8830ab0b9308f
Avoid creating FixedIntervalLoopingCall in agent UT

Set polling_interval to 0 to avoid creating LoopingCall
in loadbalancer agent tests, dhcp agent tests, linuxbridge agent tests

Change-Id: Ib6edbc11feba402a4c2f19b7d6833b49879ee626
Closes-Bug: #1288079
Merge "NSX: lower the severity of messages about VIF's on external networks"
Merge "Avoid creating FixedIntervalLoopingCall in agent UT"