add '-u USER' to restrict the process list to processes executed

Sometimes i want to know if there are processes executed as specified user.
Changes allow to do it, for example:

```
[root@localhost mcollective]# mco process list -u rabbitmq

 * [ ============================================================> ] 1 / 1

   localhost.localdomain

     PID      USER         VSZ            COMMAND                                                     
     6702     rabbitmq     10.570 MB      /usr/lib64/erlang/erts-5.8.5/bin/epmd -daemon               
     7692     rabbitmq     576.332 MB     /usr/lib64/erlang/erts-5.8.5/bin/beam -W w -K true -A30 -P 1
     7766     rabbitmq     10.535 MB      inet_gethost 4                                              
     7767     rabbitmq     12.590 MB      inet_gethost 4                                              


Summary of The Process List:

           Matched hosts: 1
       Matched Processes: 4
           Resident Size: 9.893 MB
            Virtual Size: 610.027 MB


Finished processing 1 / 1 hosts in 90.63 ms
```