Switch over to FixedIntervalLoopingCall

LoopingCall has been renamed (without a change) to
FixedIntervalLoopingCall in https://review.openstack.org/#/c/26345/

Adjust to the new name so that the compatibility code can be removed.

Change-Id: I64f62dca5bee580dcd28860760208e9be8f4b659
Closes-Bug: #1307560
Merge "Switch over to FixedIntervalLoopingCall"