Add support for Eucalyptus endpoints

- Add facade for constructing AWS service clients w/ configured endpoints.
- Add properties for setting AutoScaling and CloudWatch endpoints.
  com.netflix.servo.aws.endpoint.cloudwatch
  com.netflix.servo.aws.endpoint.autoscaling
- Construct clients using facade in CloudWatchMetricObserver and
  AwsInjectableTag.
Merge pull request #189 from eucaflix/eucalyptus

Add support for Eucalyptus endpoints