Correction of tempest verification

1) Revert direct usage of subunit

Test repository should be used as launcher for tempest tests, since direct
usage of `subunit` broke execution of `compute` set as well as `full` set.
Tests discovery and execution can't be split at the moment.

2) Cleanup of tempest folder afterfaulty installation

Also, execution of tempest sets cannot be correct without confidence in
correct installation of tempest repository. Cleanup of tempest folder after
faulty installation should be added to fix check of tempest installation.

3) Length of database field for verification results

MutableJSONEncodedDict limited by 64k. This length is not sufficient for
storage results of those verifications, which have a huge number of failures
and tracebacks.

4) Incorrect usage of mock in tests

Class `mock.MagicMock` doesn't have method `assert_called_once`.
Instead of this one, method `assert_called_once_with` should be used.
Based on the fact that this is critical error in testing, this patch
affects not only tempest verification.

Change-Id: I68660e38f2cae3f60a01e99cdbbb57bdb8a9bb04
Closes-Bug: #1313744
Closes-Bug: #1313742
Closes-Bug: #1305991
Merge "Correction of tempest verification"