Made running OSTF tests easier

* improved run_tests.sh and tox.ini (they are like those in
  fuel-web now):
  * it is possible to run unit and integration tests in
    various combinations
  * tox is used to run tests
  * added xunit reporting
  * ostf-server for integration tests is run and killed by
    run_tests
* introduced requests_mock for mocking http requests
* removed not used code that was responsible to be used
  as a fake nailgun
* functional tests are not included in run_tests.sh for now
* switched from pep8 to flake8
* moved some tests to new category: integration tests
* added "base test cases" - for unit & integration tests
* made unit tests not require root access, DB access and HTTP
  requests

Change-Id: Id475b3f38d49eaa02012d16a9cc8caf09d5210d8
Closes-Bug: #1404892