[bosh_agent] Fix dhclient3 problem when restarting network

If dhclient3 cannot release a lease because it collides with a network
restart (message "receive_packet failed on eth0: Network is down"
appears at /var/log/syslog) then the old dhclient3 process won't be
killed (see bug LP #38140), and there will be two dhclient3 process
running (and dns changes will be flip floping each lease time). So
before restarting the network, we first kill all dhclient3 process.