Fix NoSuchOptError in lbaas agent test

Option 'periodic_interval' doesn't exist when using testtools
to test lbaas agent, fix this by adding a mock object.

Change-Id: I5ef02d6fa154ea7a18ca77d02a2704b7e3990c3b
Closes-Bug: #1264260
Merge "Fix NoSuchOptError in lbaas agent test"