Read standard environment variables for host, user & token

[claytonrcarter]

I'm new to go, but excited about this utility. Thanks for creating it!

I was reading the config code and it a looks like lab will use LAB_CORE_HOST, ..._USER and ..._TOKEN when launching. I can't find specific documentation at GitLab about this, but a number of other projects use the GITLAB_API_ENDPOINT and GITLAB_API_PRIVATE_TOKEN variables for this. (GITLAB_API_USER seems less common, but could also be used.)

What do you think about having lab read these variables as well?

[claytonrcarter]

Actually, I did some more research and it looks like GITLAB_USER_LOGIN has been in CI used since GL 10 to hold "The login username of the user who started the job" (https://docs.gitlab.com/ee/ci/variables/) and might be an already-supported-but-not-for-this-exact-use-case variable.

[zaquestion]

Very open to suggestions here and on the overall config format. I'm planning to give the whole thing an overhaul for 1.0

Mainly to support #151 -- I'm inclined to hold off on more environment variables until then, since it may change things, but I don't feel strongly about that