support authentication from OS_* environment variables

[thomaslaurenson]

Firstly, very cool project - I have been using a local build against our OpenStack environment. However, one issue for us is that most team member using the standard OS_* environment variables, either sourced from an openrc file or injected per process by a secret manager - I have a password store extension I use (https://github.com/thomaslaurenson/pass-env).

Changes

• When OS_AUTH_URL is set, the Contexts screen lists an extra context named envvars (name taken from openstacksdk)
• Activating envvars authenticates with gophercloud from the OS_* variables; everything downstream sees an ordinary context
• Startup no longer requires a clouds.yaml when OS_AUTH_URL is set; without either, the error and exit code are unchanged
• README: new "Environment variables" subsection under Usage plus one sentence in the plugin notes (couldn't find a good place for this, but seems to work)

Limitations

• As with clouds.yaml, user and project are assumed to be in the same domain; use OS_PROJECT_ID otherwise
• OS_CACERT, OS_CERT and OS_KEY are not applied to this context yet

This was a first effort to add envvar support. I did some testing locally with envvars set, and clouds.yml files in default locations. Open to any feedback and changes, and have "allowed edits by maintainers".

[akyriako]

Hej @thomaslaurenson, thanks a lot taking an interest in the project and taking the time to contribute as well. The OS_* env var support is a very useful addition, and I really appreciate the effort you put into the implementation, testing, and docs.

Which other OpenStack services are you using regularly? and are there particular commands/actions you’d like to see implemented in o7k? Real-world feedback in this phase is extremely useful for figuring out where to take the project next.

Thanks again and feel free to ⭐ the repo and spread the word so more OpenStack folks get to know the project.