Rackspace Cloud DNS

[ianw]

Support Rackspace Cloud DNS

This commit is based on the original pull request by tcocca
#1297

Addtional cleanup was provided by senseisimple in
#1999

This pull request has squashed the changes for review, rebased, fixed a minor (but breaking) problem with the field ordering in the response, and added documentation per the API guide.

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I have been through the DNS API dev guide and believe this is ready:

1. cloudflare as reference : very similar
2. script name : is consistent
3. location : dnsapi/ for inclusion
4. #!/usr/bin/env sh : yes
5. has dns_myapi_<add|remove> functions: yes
6. add function guides:
   6.1. gets fulldomain/txtvalue in dns_myapi_add(): yes
   6.2. save credentials in _add() : yes, via _rackspace_check_auth()
   6.3. detect root zone: yes, via _get_root_zone
   have tested
   example.org
subdomain.example.org
subsubdomain.subdomain.exmaple.org
   6.4. use _post/_get : yes, in _rackspace_rest
   6.4.a. wildcard txt certs: tested with *.example.org
   6.5 process strings using functions, no awk: yes
7. rm function runs standalone: yes, records are removed

[Neilpang]

please create a new issue for bug reporting:
https://github.com/Neilpang/acme.sh/wiki/DNS-API-Dev-Guide#10-please-create-a-new-issue-for-future-bugs

[ianw]

@Neilpang done with #2091

(sorry, didn't know if this was done after or before merge)