Make the default test environment configurable.

[mbruzek]

The review queue automatically runs the LXC/LXD tests for each submission.

Our container charms will not work on LXD so that test will almost always fail. I would like to be able to select a different substrate to test against by default.

Thanks,

[arosales]

Also be good to run the test on at least on major cloud (AWS, Azure, OpenStack or GCE), and architectures we support (ppc64el and s390x). Ideally we get to a point where we always run across these substrates automatically. For now +1 to running at least on one public cloud in addition to LXD.