Add Plugin.do_path_regex_sub()

Add a method to the Plugin class to apply a regex substitution
to a set of paths maching a path regex. For e.g.:

    self.do_path_regex_sub(r'/etc/foo.*', 'pw=(.*)', 'pw=****')

The oVirt plugin will use this.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>