Keep drain logic optional

[prashanth26]

What would you like to be added:
Keeping the CA drain logic optional

Why is this needed:
The MCM controller already has a drain logic. So the idea would be to explore the possibility of keeping CA drain logic optional and disable it in the gardener's use case.

[himanshu-kun]

Post grooming discussion

• CA drain is called before DeleteNodes() (cloudprovider implemented method) is called , so we can't influence CA drain through our implementation
• For fixNodeGrpSize() , no CA drain is called , possibly because it deals with nodes which won't have any pods on them
• CA drain evicts all pods without distinguishing b/w PV and non-PV pods in parallel. It has not caused us any issues, because its called mostly in scale-down , and in that case not many pods with PV are there.
• We need to look into how ClusterAPI deals with this problem.

[himanshu-kun]

An upstream issue to track this -> kubernetes#6335

[ashwani2k]

This will be dealt with CA-MCM overhaul.