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.