qa: Add a tox env that can test importing files

qa: Add a tox env that can test importing files

While switching to python3, we need to make sure that we can import
the qa/tasks (and others, but this starts with qa/tasks) on a python3
environment.
To test this, we need to install teuthology into the test
venv. Currently, teuthology is not py3 ready so this will fail.

To test the current state of the qa/tasks directory with the ongoing
work for python3 within teuthology, you can now do:

TEUTHOLOGY_GIT=git+https://github.com/kshtsk/teuthology.git@wip-py3-compat \
    tox -eimport-tasks

This is using the current branch from
https://github.com/ceph/teuthology/pull/1362 which does the work to
make teuthology python3 ready.

NOTE: This tox env is not activated by default. It's currently failing
but it provides a way to iterate over the failures and once we have
them fixed, we can activate the tox env during make-check.

Signed-off-by: Thomas Bechtold <tbechtold@suse.com>