Enable connection draining on UAA ELB
Enable connection draining on UAA ELB

Previously the ELB would close all in-flight requests to the router when
gorouter was failing healthchecks because it is in draining mode.

Enabling this allows those requests to complete and reduces the number of
failed requests that we see when deploying a stemcell change to the `router`
VMs.

From approximately 22 failed requests like:

    2017-11-14 11:27:24.20 UTC - 2017-11-14 11:27:26.06 UTC Fetching detailed app information: Error getting token: Post https://uaa.dcarley.dev.cloudpipeline.digital/oauth/token: EOF
    2017-11-14 11:27:25.50 UTC - 2017-11-14 11:27:26.07 UTC Fetching detailed app information: Error getting token: Post https://uaa.dcarley.dev.cloudpipeline.digital/oauth/token: EOF
    2017-11-14 11:27:21.84 UTC - 2017-11-14 11:27:26.07 UTC Fetching detailed app information: Error getting token: Post https://uaa.dcarley.dev.cloudpipeline.digital/oauth/token: EOF
    2017-11-14 11:27:28.47 UTC - 2017-11-14 11:27:30.10 UTC Fetching detailed app information: Error getting token: oauth2: cannot fetch token: unexpected EOF

To approximately 11 failed requests like:

      2017-11-14 12:29:48.26 UTC - 2017-11-14 12:29:51.27 UTC Fetching detailed app information: Error getting token: oauth2: cannot fetch token: 503 Service Unavailable
      2017-11-14 12:33:12.41 UTC - 2017-11-14 12:33:15.42 UTC Listing all apps in a space: Error getting token: oauth2: cannot fetch token: 503 Service Unavailable

A following commit hopes to reduce these errors to 0.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf