[rackspace] Don't parse JSON in delete_server

The Rackspace API replies with an empty response to delete_server
requests, but includes a header enforcing the application/json
content-type.

This commit disables JSON parsing on delete_server requests,
which prevents the appearance of these warnings:

[WARNING] Error Parsing response json - 781: unexpected token at ''
Merge pull request #2163 from DavidWittman/rs-delete-server-json

[rackspace] Don't parse JSON in delete_server response