It seems at some point oslo_service loopingcall started using
eventletutils from oslo_utils to sleep during the loopingcall
retries, and some untittests started taking up to 40 seconds
to complete. This change mocks out the correct method offering
significant speedup to unittests' run time.
The EventletEvent class is introduced to eventletutils in version
3.38.0 so lower constraints are bumped as well.
Change-Id: Id7e6ff2a4748b5301e2259acdc760ac7f56b96c3
68a5e31a54