Add log cache instance group

[acrmp]

• Log Cache has been split out to its own instance group: https://github.com/cloudfoundry/cf-deployment/releases/tag/v18.0.0
• Remove stale reference to etcd

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

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

[mjgutermuth]

Thanks @acrmp!