[fix-centos-63] only try to kill a process that exists

This fixes the first run convergence under centos-63.

the stack trace looks like,

```
[2013-02-15T23:14:15+00:00] INFO: Retrying execution of execute[kill_collectdmon], 5 attempt(s) left
[2013-02-15T23:14:17+00:00] INFO: Retrying execution of execute[kill_collectdmon], 4 attempt(s) left
[2013-02-15T23:14:19+00:00] INFO: Retrying execution of execute[kill_collectdmon], 3 attempt(s) left
[2013-02-15T23:14:21+00:00] INFO: Retrying execution of execute[kill_collectdmon], 2 attempt(s) left
[2013-02-15T23:14:23+00:00] INFO: Retrying execution of execute[kill_collectdmon], 1 attempt(s) left
[2013-02-15T23:14:25+00:00] INFO: Retrying execution of execute[kill_collectdmon], 0 attempt(s) left

================================================================================

Error executing action `run` on resource 'execute[kill_collectdmon]'

================================================================================


Mixlib::ShellOut::ShellCommandFailed

------------------------------------

Expected process to exit with [0], but received '1'
---- Begin output of kill -TERM $(pidof collectdmon) ----
STDOUT: 
STDERR: kill: usage: kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigspec]
---- End output of kill -TERM $(pidof collectdmon) ----
Ran kill -TERM $(pidof collectdmon) returned 1


Resource Declaration:

---------------------

# In /tmp/vagrant-chef-1/chef-solo-1/cookbooks/collectd/recipes/_install_from_package.rb

 27: execute 'kill_collectdmon' do
 28:   command 'kill -TERM $(pidof collectdmon)'
 29:   user 'root'
 30:   action :nothing
 31:   retries 6
 32: end
 33: 

Compiled Resource:

------------------
# Declared in /tmp/vagrant-chef-1/chef-solo-1/cookbooks/collectd/recipes/_install_from_package.rb:27:in `from_file'

execute("kill_collectdmon") do
  action [:nothing]
  retries 0
  retry_delay 2
  command "kill -TERM $(pidof collectdmon)"
  backup 5
  returns 0
  user "root"
  cookbook_name :collectd
  recipe_name "_install_from_package"
end
```