feat: support for variable only deployments

[shreddedbacon]

This change allows for a variable to be set when triggering a deployment that will run a variables only deployment.

This will only update the secret where the variables are stored, and restart pods. This means only variables that are RUNTIME or GLOBAL scope will be modified.

If a GLOBAL scope variable has a legitimate build requirement, then this will go unchanged. If you have a GLOBAL scope variable that when changed requires a build step, you should perform a full deployment. If you're only using the GLOBAL scope because you're unsure if a variable is used in a build, chances are it isn't, consider changing to RUNTIME

This will return an error if the user tries to apply variables only changes on an environment that still has a configmap for the variables, a full deployment is required to ensure everything gets updated correctly before running a variables only deployment.

We may need to consider other things to check, for example if some things have changed in the upstream git repo that may result in odd behaviour when the generator runs.

[bomoko]

Have run multiple tests of this and it seems to work as expected