Use shared credential information from gscloud

[nvthongswansea]

Ref #124. Changes:

• Add config_file provider variable.
• Add config_file to docs.

Expected behaviour:

• If uuid and token cannot be found in either environment variables or tf definition files, the gscloud's config file is used to import uuid and token. The path of the gscloud's config file can be set, or the default path will be used. Only the first configured account in the config file is used.

Usecase:

• If the user already uses and configured gscloud on his machine, he doesn't need to configure the credential information when using terraform-provider-gridscale. He just needs to install terraform and terraform-provider-gridscale, and use right away.

[bkircher]

Sorry for late response.

I'm missing a little bit the "farsightedness" in this PR.

So TF should not have anything to do with gscloud, and shoud not read the config files that gscloud provides.

What we want, I think, is something much like how AWS command line utilities and libraries are configured.

There is a distict set of config files that are tool and library agnostic.

Again, maybe open an GH issue first where you discuss what files, what locations, the schema and ask people from the Python lib, the Node.js client, gscloud, ... to hop in and give there input if that would work for them.

[nvthongswansea]

Close due to further discussion.