uniter: added opened-ports hook tool

Final step in enabling support to open and close port
ranges from charms is the new "opened-ports" hook tool.
It returns a sorted list of all port ranges opened by
the current unit, like this:

10-20/tcp
80-90/tcp
100-200/udp
8080/udp

As a side-effect, this patch also fixes the following
bug: http://pad.lv/1216644