write-ifcfg.sh: add UUID=.. and save the lease files with the same uuid

write-ifcfg.sh: add UUID=.. and save the lease files with the same uuid

As described in https://bugzilla.redhat.com/show_bug.cgi?id=541410#c2,
if you want NetworkManager to take over an interface that you're using
for NFS root (or other network root device), you need to:

a) set UUID=<uuid> in ifcfg-<iface>, and
b) save the lease file as /var/lib/dhclient-<uuid>-<iface>.lease

This patch should make write-ifcfg handle both these things.