Allow specifying --port-range-begin and --port-range-end

Options ``--port-range-begin`` and ``--port-range-end`` would allow customization of the ports range used by ``portallocator``, and they keep current defaults specified by constants.
**NOTE**: I have wrapped some globals to make explicit initialization a prerequisite.

Docker-DCO-1.1-Signed-off-by: Giuseppe Mazzotta <gdm85@users.noreply.github.com> (github: gdm85)