Tox file for testing
Adding tox.ini file as an example for [the discussion](https://github.com/napalm-automation/napalm/issues/327).
Coverage remained the same at 79.901% when pulling **64a12378a9f56ac4f32e165dc73be73bbd1d7a8d on ogenstad:tox** into **36e24de2622a055820d434762cb24ce5af780136 on napalm-automation:develop**.
I like it, not sure if we need something more here, do you know if this is gonna have some impact on coveralls.
Anyway, we can probably test. If you do the changes I mentioned in the code, add `.tox` to gitignore if it's not there already, and update `.travis.yml` we can see how it behaves.
Btw, what's the implication on travis? Now we tell travis it has to run on different python version, do we have to just leave one and let travis handle it?
Googling it seems the correct way is using; `tox-travis`. Do you mind playing with it? : )
I think the use of tox-travis is mostly to show the correct Python versions on Travis instead of showing the tox environment version of Python. Compare [this](https://travis-ci.org/napalm-automation/napalm-eos/builds/184164810) to [this](https://travis-ci.org/networklore/nelsnmp/builds/153480049), might be other things that I'm not aware of. Anyway I used tox-travis for this.
I also changed setup.cfg to have pytest call pylama instead of running pylama. The reason for this is that the tests will fail as soon after the first command has a non normal exit. So pylama will never run if the other tests are failing. This way linting errors will be shown along with the results from pytest.
Coverage decreased (-0.2%) to 79.74% when pulling **cc5b9e830c331af11ce9f8d09f2cca975801611d on ogenstad:tox** into **36e24de2622a055820d434762cb24ce5af780136 on napalm-automation:develop**.
Works for me, let's see what the rest thinks.
I am merging this one and creating issues to do this on all the repos.