Allow setting an ST2_API_KEY for st2chatops instead of user/pass

[cognifloyd]

Whenever I setup chatops, I prefer giving chatops an api_key instead of a username/password combo.

This PR allows us to add a previously generated st2 api_key as st2chatops.api_key in values.yaml (make sure to add the api_key object in st2.apikeys as well). Defining this api_key triggers replacing the ST2_AUTH_USERNAME and ST2_AUTH_PASSWORD environment vars with ST2_API_KEY but only for the st2chatops deployment.

[cognifloyd]

OK. I think I found a clean enough way to make st2chatops use an api_key. What do you think?

[arm4b]

Nice 👍
Thanks a lot.

BTW, I also remember that using ST2_API_KEY is more stable and recommended comparing to st2 username:password pair as st2chatops doesn't need to re-generate auth token (which sometimes fails on the javascript side).