BUG-8494: Cleanup clustering-it-provider

Fixes various warnings and refactors MdsalLowLevelTestProvider
to be slightly cleaner in terms of number of classes.

It also eliminates synchronous thread blocking on future collection
and instead schedules task which performs the cleanup if the system
gets stuck.

Change-Id: I657f3df60c620284538bdf39ab1536eac8448801
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit d97061af6814ad7b085af10797a252aa4aa5cda6)