Add st2 login test

[Mierdin]

This adds a test case for st2 login command and subsequent commands using it's provided authentication configuration.

I have validated that this detects the current st2 login failure, and also that it succeeds when applying the fix in StackStorm/st2#3226. See full output here

[Kami]

So far LGTM, but I think we also need a set up phase which ensures the environment is clean and we start from a clean consistent start before each run:

1. Removes ~/.st2/config (set up or test should write config we want to test, we want to ensure there is no default config messing up with the tests)
2. Remove ~/.st2/token* (to ensure there are no cached tokens on the disk)

[Kami]

@Mierdin bump, how is this looking, can we merge it?

[Mierdin]

Closing this for now, as this is a bit stale. Feel free to pick up where I left off on this branch.