Accept DCE version configuration

Proposed changes

Accept DCE version configuration, as:

Also:

Accept most other params for dce system deploy as either CLI flags, YAML, or env vars (region, namespace, tfvars)
Add integration testing for dce system deploy command
Some refactoring around dce system deploy config parsing
- We a number of different structs holding different types of configuration. We also had a number of places across the code where defaults were set. I tried to move this all into a single configuration struct, and implement defaults in a single location.
Fix some functions to return errors instead of panicking (mostly in cases where it made testing difficult)
Fix a bug where CLI flags/YAML/env vars were ignored, if a cached version of main.tf already existed
- End result is that our --use-cached flag no longer has any effect. We could reimplement some caching here in the future, but it probably makes more sense around file downloads vs main.tf file generation.
Fixed bad logic for "local" DCE system deployments. Though I'd recommend that we test and harden this behavior more thoroughly before documenting for general use.

And a couple other things, while I was trying to fix unit tests:

[x] Bugfix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Refactor (changes to code, which do not change application behavior)

[x] I have filled out this PR template
[x] I have read the CONTRIBUTING doc
[x] I have added automated tests that prove my fix is effective or that my feature works
[x] I have added necessary documentation (README.md, inline comments, etc.)
[x] I have updated the CHANGELOG.md under a ## next release, with a short summary of my changes