feat: support atlantis.yaml without defined projects

[csainty]

I'd like to propose supporting atlantis.yaml config without having to specify the projects section. Enabling, for example, to set parallel_plan while still having atlantis determine the project list.

I totally understand if this doesn't fit with your own ideas, so there are no hard feelings if you disagree and close.

The code gets a bit fiddly, and I am not Go expert, but this looks like to relevant section to change, there just may be side-effects I am unaware of.