Discovering refactoring

Discovering logic has been changed in order to perform some
optimisation. Now discovering process takes place at the time of
ostf-server startup and it loads to db all existing tests.
Also some data from db (test set and corresponding
to them tests data) is cached to provide fast access to this test
repository in future. Further discovering for particualar cluster is
done by adding to database some information that links needed test sets
and tests with cluster. Linking process uses data from cache so
discovering takes less time to be performed.

Change-Id: Ib8ce2c109a216e1e704d2050baaca71f3ad367ad

Discovering refactoring / non major fixes and tests refactoring

For TestRun class from models relationship for test sets via
association_proxy() function was established. It is need because of some
cleaning actions that are performed in standalone TestRun methods by
nose plugin which consume data that is stored in test sets objects.
Unit tests (except those for REST controllers) were updated.
Some additional non-major refactoring was performed.

Change-Id: I94ef205ee6d90426929746573c836f4de7cc7b24