Support maxTargets and maxParallel with % values

[maruina]

The ProgressiveSync CRD is designed as to support maxTargets and maxParallel as IntOrString.

Consider the following scenario, when you have 4 clusters and a ProgressiveSync similar to

[...]
stages:
  - name: one cluster
     maxTargets: 1
     maxParallel: 1
     targets:
       clusters:
         selector:
           name: cluster-one
  - name: everything else
     maxTargets: 100%
     maxParallel: 25%
     targets:
       clusters:
         selector: {}

In the second stage, we would expect maxTargets to be 3, but instead we set it to 4. This is because the scheduler looks at every application matching the label selector - in this case all of them.

We need to change this logic so when we express maxTargets and maxParallel as %, they are scaled against the remaining clusters.