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
Make test_nvp_sync.SyncLoopingCallTestCase reliable

This patch removes the previous check that verified the mocked
looping call had an exact number of calls, and replaces it with a
simpler check that just verifies that the mock has been called.

As a consequence, this patch also shortens the test duration.

Closes-Bug: 1241969

Change-Id: Ia7bc66e1f2f631e46ddcbe0eacc1d5b2481274cb
(cherry picked from commit 4e311e66deb93b459527bd5e9f203720a8e28463)
Merge "Make test_nvp_sync.SyncLoopingCallTestCase reliable" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable