Use pifofproc in wait_for_if_running

pidof doesn't work for ruby scripts (e.g. chef-client)

Also removed a couple of unneeded wait_for_if_running calls in
prepare_cloudupgrade() and get rid of the killall chef-client call which
actually could cause races (e.g. blocking_chef_client.sh automatically
restarting chef-client after the "wait_for_if_running chef-client").