1136504 - added tab completion for file paths

As @rbarlow requested now tab completion for pulp-admin provides support for completing local file names. 
It works for options 
--feed-ca-cert 
--feed-cert 
--feed-key 
--gpg-key 
--host-ca 
--auth-ca 
--auth-cert

If I forgot some option which takes a full path to a file name as a value, let me know please.