Support scaling EDS down to 0 instances

[otrosien]

Expected Behavior

When updating an EDS without auto-scaling, one should be able to specify desired replicas to 0 in order to drain all data from an EDS.

Actual Behavior

The EDS change is being rejected by the verification of the manifest.

Steps to Reproduce the Problem

1. Create an EDS with scaling.enabled=false and replicas=1
2. Update the EDS to set replicas=0

[otrosien]

Idea: offer via a "allowScaleToZero" property on the EDS.