Cleanup managenement via system signal handling

Now cleanup is executed alongside with testrun code in subprocess. This
approach is more flexible and logical. To properly trigger cleanup
action signal mechanism was implemented. Also code which manages
restarting of test runs were revisited in order to tune it for
supporting new changes.

Change-Id: I5bec9cc3433b1126fb85201d9f6e5b59b29f50ce
Add test_set dependency to models

Field "exclusive_testsets" was added to TestSet entity
in models.py. This data defines test sets hierarchy for building test
runs execution chains. New migration version was generated for new data
scheme. Fixture tests were updated with purpose to support new logic

Change-Id: I654ed7edeadad47164644b2ed580a103e26c111e
Implements: blueprint ostf-adapter-testruns-orchestrator