Collapse daemon's main function

With three ganeti daemons, and one or two more coming, the daemon's main
function started becoming too much cut&pasted code. Collapsing most of
it in a daemon.GenericMain function. Some more code could be collapsed
between the two http-based daemons, but since the new daemons won't be
http-based we won't do it right now.

As a bonus a functionality for overriding the network port on the
command line for all network based nodes is added.

Signed-off-by: Guido Trotter <ultrotter@google.com>