…field to HumioUpdateStrategy

EnableZoneAwareness is true by default.

When EnableZoneAwareness is true:
- pod replacements will be performed one zone at a time.
- if no zone has been decided upon yet, pick the first one from the pods that needs replacing.
- as soon as no more pods are found in the current zone that needs replacing, the zone marker is cleared.

This also refactors the uses of os.Getenv() to helper functions to
reduce the number of places we refer to the env var names.