fix(google): do not pin source server group capacity in red/black

[maggieneterval]

Closes spinnaker/spinnaker#4812

• When deploying to AWS, only the rolling red/black strategy pins the source server group's min capacity to its desired capacity (see AwsDeployStagePreProcessor and this conversation for details on how/why).
• This change brings the GCE implementation of red/black to parity with the AWS implementation: we will no longer pin the source server group's min capacity to its desired capacity. The potential drawback to this is that during the period when both the source and target server groups are taking traffic, the autoscaler may scale down the source server group as it will only be taking 50% of the traffic: in the case a rollback is necessary, it will need to scale back up. However, as Netflix has experienced, the potential downsides of pinning the source server group are much worse, as there are many unpredictable ways to get into a state where the server group is never unpinned (see Netflix post-mortem here).
• The other upside of bringing the GCE red/black to parity with the AWS red/black is the maintainability of the common strategy code, which @marchello2000 is currently refactoring.
• In the next Spinnaker release (1.17), I will summarize the rationale behind this change in the release notes so that any GCE users relying on the existing functionality are aware of the change.

[plumpy]

Deleted code is the best kind of code!

🥇