nuke: Use pkill -KILL to unconditionally wipe out hadoop processes

nuke: Use pkill -KILL to unconditionally wipe out hadoop processes

In CentOS 7, the command "ps -ef | grep 'java.*hadoop' | grep -v grep | awk
'{print $2}' | xargs kill -9" produces undesirable output when no matching
processes exist.

Note: the "-f" option to pkill mimics the semantics of "ps -ef". For example,
"ps -ef | grep 'java.*hadoop'" will match a process called "sh java343hadoop",
while pkill will match that process only with the -f option:

$ ps -ef | grep "java.*hadoop" | grep -v grep
root     16165  4101  0 11:57 pts/1    00:00:00 sh java343hadoop
$ pkill -KILL "java.*hadoop"
$
$ ps -ef | grep "java.*hadoop" | grep -v grep
root     16165  4101  0 11:57 pts/1    00:00:00 sh java343hadoop
$ pkill -f -KILL "java.*hadoop"
[1]+  Killed                  sh java343hadoop

Fixes: http://tracker.ceph.com/issues/17981
Signed-off-by: Nathan Cutler <ncutler@suse.com>