Delivery Service XML IDs should be limited to lower-case letters

The DNS system is case-insensitive. Since a delivery service XML ID is used as part of the FQDN of the cache being redirected to, two different DSs cannot differ only by case.
This leads to the conclusion that it is best if we limit the XML IDs of delivery services to be lower-case only.
This would achieve the following:
1. Make domain names used by TC 'conventional' (i.e. lower-case only)
2. Remove the possibility of a case-conflict between DSs
3. Currently, Traffic Router does not behave correctly when a DS XML ID contains upper case letters. Limiting to lower-case would prevent the need to fix this

Current problems with TR behaviour, when an XML ID contains opper-case letter are:
1. The TR sends a redirect to a host FQDN which contains a lower-case version of the DS XML ID
2. The TR does not resolve the lower-case version of the host FQDN.

Here is an example to demo current bug in TR. DS XML ID is opencachehub-DT, TR redirects to opencachehub-dt, and then refused to resolve the cache name using this DS (a lot of irrelevant data was removed fro this text):

$ curl -L -s -D - http://tr.opencachehub-DT.stage-cdn.tc-stage.cqloud.com/video01.mp4 -v

• Connected to tr.opencachehub-DT.stage-cdn.tc-stage.cqloud.com (54.244.152.242) port 80 (#0)
  > GET /video01.mp4 HTTP/1.1
  > Host: tr.opencachehub-DT.stage-cdn.tc-stage.cqloud.com
  > Accept: /
  >
  < HTTP/1.1 302 Moved Temporarily
  < Location: http://p39-edge-lab.opencachehub-dt.stage-cdn.tc-stage.cqloud.com/video01.mp4
  < Content-Length: 0

<

• Connection #0 to host tr.opencachehub-DT.stage-cdn.tc-stage.cqloud.com left intact
• Issue another request to this URL: 'http://p39-edge-lab.opencachehub-dt.stage-cdn.tc-stage.cqloud.com/video01.mp4'
• getaddrinfo(3) failed for p39-edge-lab.opencachehub-dt.stage-cdn.tc-stage.cqloud.com:80
• Couldn't resolve host 'p39-edge-lab.opencachehub-dt.stage-cdn.tc-stage.cqloud.com'