DEVOPS-542 allow customized endpoint label

[chessracer]

This supports the use-case where we have a rancher env spanning multiple aws accounts - we need the ability to target an aws-elb service to a specific set of services. Adding a custom label to the service, and allowing a customized label that aws-elb looks for, provides that functionality.

[chessracer]

This is not yet working as planned. The following setup fails:

• hosts in rancher env are all in same aws account
• single rancher env, with multiple stacks
• each stack has an aws-elb configured with a custom SERVICE_LABEL_ENDPOINT
  The aws-elb's do not correctly ignore services with different SERVICE_LABEL_ENDPOINT 's, and wind up deregistering eachother's instances.
  The setup does work in the original use-case, where the rancher env spans multiple aws accounts - in that scenario, the aws-elb's cannot find the other env's elb's to "deregister" them.
  This behavior should be corrected so that it completely ignores services with different SERVICE_LABEL_ENDPOINT 's .