Move gate tests under mistral/tests

- Move all gate tests from functionaltests folder to the mistral/tests/functional
- Edit run directory for gate tests in the functionaltests/run_tests.sh
- Leave 'resources' folder in the 'mistral/tests' to use examples from it in the tempest tests

Change-Id: I4d6d6a653aab5af00c5cae7c21495f4a86ebfe42