Allow forcing ownership even when using --rpm-use-file-permissions

The rpm-use-file-permissions setting asks each file who owns it and what
the mode is. Sometimes you only want the mode, or sometimes the user is
not correct for deployment. Now, specifying --rpm-user will force the
owner to be the given user but still respect the file modes gathered
by --rpm-use-file-permissions.

The intent of this change is:

* --rpm-use-file-permissions still works the same, when alone
* --rpm-user forces the 'user' owner regardless of the above
* --rpm-group forces the 'group' owner regardless of the above

This was identified by https://logstash.jira.com/browse/LOGSTASH-2012