Create CLI in python capable of fetching containers and running tests.

[tskluzac]

When you run the testing suite, the system will first check the .xtract/containers folder for the name of all 'configured' containers, and if one is missing, then we grab it via Globus from Petrel. So a user might execute the following commands:

xtract fetch-containers [library]

where [library] can be standard, materials, or all.

Users should also then be able to run Xtract tests locally for all containers present.
xtract run-tests [library]
and it will run all the tests for all of that kind of container.