Use dhclient by default

Ubuntu 14.04 is using dhclient, not dhclient3. If dhclient3 is present
configure /etc/dhcp3/dhclient.conf. Otherwise, fall back to /etc/dhcp/dhclient.conf.
Restart networking with ifup/ifdown which restart dhclient.
/etc/init.d/networking does not restart dhclient.

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
WIP ifdown version

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>