ui: Fixed input vars + git settings overwriting bug

[sabrinako]

Closes #2040 (check issue for further description of bug behavior)

How to test

1. Pull down branch
2. Start up a local Waypoint server
3. waypoint ui -authenticate and proceed to localhost to approve cert authority
4. cd ui and ember serve local
5. Go to localhost:4200
6. Test these various situations:

• Create project with an attached git repo, add input variables, edit git settings again
• Create project without git repo, add input variables
• Add, edit, delete input variables
• Try all 3 of the authentication methods settings (user, ssh, none)
• Ensure SSH keys and server-based HCL files show up as expected

[gregone]

Looks good @sabrinako. Thanks for extracting the upsert logic into the api service 👍 . I only have a question about the authcase value.