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

(cherry picked from commit 712923d3ab3dd4b0b133a11735fed1533db84995)
Change-Id: Ib6edbc11feba402a4c2f19b7d6833b49879ee626
Closes-Bug: #1288079
Closes-Bug: #1311232