Scale-up only new machineSet while scale-down all active mSs proportionally

[himanshu-kun]

What this PR does / why we need it:
This PR changes the scaling logic of machineDeployment.
#Before
a scale-up or scale-down lead to proportional splitting of the increase/decrease in replicas among the machineSets as per their size
#Now
a scale-up leads to increase in replicas of the new machineSet only, while scale-down leads to the same proportional scaling as before.

This is done keeping in mind working of autoscaler with MCM. Because during rolling update if autoscaler asked for a scale-up , it used to add machine to the old machineSet(s) during the initial phases of the rolling update. This was inefficient as that machine would be drained later leading to longer rolling update cycle.

Which issue(s) this PR fixes:
Fixes #754

Special notes for your reviewer:
The change is made keeping autoscaler interaction with MCM in mind. Corresponding PR on autoscaler is also raised, to stop autoscaler from scaling down during rolling update operations.

Release note:

If during a rolling update scale-up is done, MCM scales up only the new machineSet, while in case of scale-down the scale-down amount is split among all active machineSets (.i.e. with `.spec.replicas`> 0), in proportion to their sizes.

[himanshu-kun]

ran the IT tests. running fine.

[gardener-robot]

@rishabh-11 You have pull request review open invite, please check