Do not use pgrep when checking the process

pgrep implementation is different on ubuntu 10.04 and 14.04
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.
pkill dhclient will take care of both dhclient and dhclient3

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>