[rackspace|databases] Fixed endpoints and added accept header

This pull request fixes an issue with Rackspace Database service. 

The hard coded endpoints contained an extraneous trailing slash that was causing a user to experience a 401 unauthorized error for all database requests.

After fixing the endpoint, I was getting a python stack trace in the response body. I tracked this down to a lack of an Accept header. I believe this issue might impact more than just the database service.