Allow api_version override

This is an experimental patch that provides a caller two ways to override a request's api_version.
    
First method: In the call to a request.
    
       compute_service.request :create_server, :api_version => v1 { ... }
    
Second method: In the configuration file.
    
       production:
         provider: openstack
         auth_service:
           # declare auth service here
         compute_service:
           api_version: v2