Add application validation

[jessesuen]

When adding an application, the API server will now validate:

• the git repository is accessible
• the git path contains a valid app.yaml
• the specified environment exists
• the referenced cluster has been added to ArgoCD

[merenbach]

Looks good. Just a couple minor concerns regarding DRY and bigger-than-one's-head methods. :-D

[merenbach]

Wondering if dividing this method into separate methods called from here might help improve future maintainability (and maybe even unit testability).

[jessesuen]

I do agree in principle, but I generally prefer to do something like that on an as-needed basis to avoid function sprawl. For now the only callers that need this are application add and update, and they just want to validate the app as a whole, instead of validating different aspects.

[merenbach]

We have a function in util/cli/config.go that unmarshals local files and has unit tests. Wondering if it might be useful here for DRY. (Please see util/cli/config_test.go, cmd/argocd/commands/app.go, and util/localconfig/localconfig.go for usage examples.)

[jessesuen]

Thanks for the pointer, fixed.