Support atlantis validate subcommand

[rmccarthy-ellevation]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story
I am looking for a way to let our developers run some lightweight unit testing for the terraform they write.

Describe the solution you'd like
I would like to be able to run terraform validate like I can plan and apply. Once A PR is opened, and a plan has been generated, the user can run "atlantis validate", which will trigger terraforms validate functionality. The output is given like a plan or apply output in the PR.

Describe the drawbacks of your solution
None that I can think of right now.

Describe alternatives you've considered
We could run this locally but since we have implemented our developers don't run terraform locally.