Support top level modules folder with directory-based config

[lkysow]

Given the following directory structure:

modules/
  module1/
    main.tf
vpc/
  main.tf

If a change is made to both module1/main.tf and vpc/main.tf, Atlantis will attempt to run terraform plan in vpc/ (which is correct) and in the root of the project (which is incorrect).

This occurs due to an attempt to support this type of structure:

main.tf
modules/
  module1/
    main.tf

The way Atlantis works right now is we assume if there's a change in any directory called modules we should run plan/apply in the root.

Possible solutions:

• requiring an atlantis.yaml file in all project roots. We don't run anywhere that doesn't have one
• detecting if there are no .tf files in the parent of modules/ and not running there if that's the case

[lkysow]

Fixed by #13. Will no longer run plan in the parent dir of modules/ unless there is a main.tf in that dir.