remove HPA and make istiod replicas configurable

[XanderStrike]

• the ingressgateway one does not function, because we deploy a daemonset
• we know from previous research that hpas do not help overloaded pilot/istiods
• defaults to 1 istiod for proof-of-concept/local deployments

Co-authored-by: Clay Kauzlaric ckauzlaric@vmware.com

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/174764287

The labels on this github issue will be updated when the story is started.