netinfo: Infer 'bootproto4' just from the existence of a DHCP lease

If we found an active (not outdated) DHCPv4 lease for an interface we don't
have to check if we have an address. Most probably it will be there.
If it weren't we would not be concerned about 'bootproto4' attribute anyway.

Change-Id: Ie635f332958e717a561227a276333afe72cdbae0
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36182
Reviewed-by: Ido Barkan <ibarkan@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>