netinfo, tests: Rename internal 'dhcp4' variables to get in line with 'dhcpv6'

Change-Id: I87edfb7a962eb28b769cd57f249deb182421622e
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36037
Reviewed-by: Ido Barkan <ibarkan@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>