Separate MSPilupeObj creation and validation

[vkuznet]

Impact of the new feature
Current implementation of MSPilupeObj has validation step as part of object constructor. Therefore, the MSPileupObj constructor requires certain attributes, like validRSEs. This works fine when we create new MSPileup objects, but it is not required (and in fact redundant) if we updated MSPileupObj which we create of existing one. This requires to pass unnecessary parameters at constructor during update cycle.

Is your feature request related to a problem? Please describe.
This feature pop-up during implementation of partial pileup document, see #11807

Describe the solution you'd like
We should separate validation step from constructor of MSPileup object, and run it separately ONLY during createPileup MSPileup API in MSPileupData.py module.

Describe alternatives you've considered
Do nothing and keep passing parameters to MSPileupObj constructor.

Additional context
Please see https://github.com/dmwm/WMCore/pull/11807/files#r1455594348 and questions raised above.