Exclude terminating machines when managing machineset replicas

[takoverflow]

What this PR does / why we need it:
Excludes considering terminating machines in machineset controller when
computing the necessary machines that need to be created/removed.

A machine will only move to terminating state once the machine set has been scaled-down
hence if a machine is already in terminating state, it should be excluded from the list
of machines that are being considered when computing the active replicas of a machine set.

The only other scenario where a machine moves to terminating is when its in Failed state
and the MCS controller proceeds to consider it as a stale machine and marks it
for removal. In this case, by excluding the terminating machine from diff computation,
we ensure that a replacement machine is spawned for the failed machine.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Release note:

Exclude terminating machines when managing machineset replicas to allow for faster replacement of a failed machine.

[gardener-prow]

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[thiyyakat]

Hi! Thanks for the PR. Just one small suggestion. PTAL.

[thiyyakat]

/lgtm

[gardener-prow]

LGTM label has been added.

Details

Git tree hash: 340a0ae2f8c56893fbdfa779348d597536173d23