[PROPOSAL] Allow configuration of multiple webhook tokens

[jaxxstorm]

Currently, atlantis only supports a shared token which presents a secret separation problem when using atlantis across multiple teams.

It would be unwieldy and difficult to build an RBAC solution into atlantis, but I believe a relatively small change could help with this.

Currently, both the gitlabtoken configuration and github token configuration consists of a string which is read from an environment variable or a command line flag. This environment variable is a string which is compared with the string from the webhook request.

Changing the string flag to be a stringslice (allowing multiple tokens to be provided) then changing the following validation comparison here to check if the headerstring is in the secret string slice could allow for further configurability.

A further enhancement could be to reach the name of the secret environment variable from the atlantis.yaml file. For example, inside a repo you could have an atlantis.yaml file like so:

version: 2
automerge: true
secret_env_var: MY_SECRET_ENV_VAR

Then, when you start the atlantis server, you could allow the user to provide arbitrary environment variables and do a comparison on those.

I may be able to find some time to push the former enhancement, but I'm not familiar enough with the code base to do the latter, although I do believe it would be very helpful.

[Jaff]

Our team would welcome a similar approach, since our terraform repo contains multiple accounts and projects, all sharing a common modules tree.