Refactored repo delete command to use the polling base class

I may still tweak the base command output, but that shouldn't affect the code in this request. This also contains some fixes to python variable shadowing (id).

New output:

$ pulp-admin rpm repo delete --repo-id doomed
This command may be exited via ctrl+c without affecting the request.

Waiting to begin...
[-]

Repository [doomed] successfully deleted