1136504 - added tab completion for file paths

https://bugzilla.redhat.com/show_bug.cgi?id=1136504

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. 
I tried this in CentOS 6, and I got this error:

```
[rbarlow@notepad ~]$ pulp-admin rpm repo create --feed-key -bash: _filedir: command not found
-bash: _filedir: command not found
```

Is _filedir something new in more recent version of bash?