tox: Split tests into separate tox environments

tox: Split tests into separate tox environments

* Add argument parsing to run_tests.sh so tests can be selected
* Define tox environments for:
  * Coverage
  * Unit tests
  * Functional tests
    * for py27 and py3
* Add environments to default env
  * Behaviour is the same when the tests are run
* Updates the py27 environment config, and adds a py3 config.
  * Commands from testenv are moved to py27
  * py3 env uses the same commands, but runs with python3

Change-Id: I22f17c0103101f01ec268b1de9d146780ae67a6d
JIRA: YARDSTICK-837
Signed-off-by: Emma Foley <emma.l.foley@intel.com>