Allow request 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

Change-Id: Ic951a3a222eecc250219cb7ec891f038f7e167ea