Rename configuration variable GATEWAY to KVMHOST

Rename configuration variable GATEWAY to KVMHOST

A previous commit meant that the value of GATEWAY is no longer used as
the gateway for the default network route, so renaming the variable
adds clarity.

This means that GATEWAY is no longer available to kickstart via the
basic post-install script.  So generate some new variables INSTALL_GW
and INSTALL_IP in create_base() and tweak the post-install logic to
use them.

Signed-off-by: Martin Schwenke <martin@meltin.net>