Automatic merge from submit-queue

AWS: More ELB attributes via service annotations

Replaces #25015 and addresses all of @justinsb's feedback therein. This is a new PR because I was unable to reopen #25015 to amend it.

I noticed recently that there is existing (but undocumented) precedent for the AWS cloud provider to manage ELB-specifc load balancer configuration based on service annotations.  In particular, one can _already_ designate an ELB as "internal" or enable PROXY protocol.

This PR extends this capability to the management of ELB attributes, which includes the following items:
* Access logs:
    * Enabled / disabled
    * Emit interval
    * S3 bucket name
    * S3 bucket prefix
* Connection draining:
    * Enabled / disabled
    * Timeout
* Connection:
    * Idle timeout
* Cross-zone load balancing:
    * Enabled / disabled

Some of these are possibly more useful than others.  Use cases that immediately come to mind:

* Enabling cross-zone load balancing is potentially useful for "Ubernetes Light," or anyone otherwise attempting to spread worker nodes around multiple AZs.
* Increasing idle timeout is useful for the benefit of anyone dealing with long-running requests. An example I personally care about would be git pushes to Deis' builder component.