More explicit failure indication in cbt run.

When executing the cbt.py test suite, it is very hard to figure out which steps failed/passed.
My experience with this tool is very limited as I just started using it, but I see that the pdsh commands fail without any error, so it is hard to decipher why.

Also, the use_existing flag in cluster: configuration in the yaml file should be highlighted when using against an existing cluster. Once I go through a successful execution I will create a pull request for any doc changes if makes sense and other issues if I see.

Another issue I see is username and groupname are taken as the same which is not the case. Might be useful to add a groups filed as well.

Lastly -->
Now I think I have gotten past some of my inital hurdles and am able to execute an fio benchmark, but I am not sure what is next.

The last step I see is:

`21:30:37 - DEBUG - cbt - pdsh -R ssh -w behzad_dastur@v-stagemon-002-prod.abc.xyz.net,behzad_dastur@b-stageosd001-r19f29-prod.abc.acme.net,behzad_dastur@v-stagemon-001-prod.abc.acme.net sudo chown -R behzad_dastur.behzad_dastur /tmp/cbt/00000000/LibrbdFio/osd_ra-00004096/op_size-01048576/concurrent_procs-001/iodepth-064/randwrite/*
21:30:37 - DEBUG - cbt - rpdcp -f 1 -R ssh -w behzad_dastur@v-stagemon-002-prod.abc.acme.net,behzad_dastur@b-stageosd001-r19f29-prod.abc.acme.net,behzad_dastur@v-stagemon-001-prod.abc.acme.net -r /tmp/cbt/00000000/LibrbdFio/osd_ra-00004096/op_size-01048576/concurrent_procs-001/iodepth-064/randwrite/* /tmp/00000000/LibrbdFio/osd_ra-00004096/op_size-01048576/concurrent_procs-001/iodepth-064/randwrite
`

I can see logs created at:

`[root@cbtvm001-d658 cbt]# ls /tmp/00000000/LibrbdFio/osd_ra-00004096/op_size-01048576/concurrent_procs-001/iodepth-064/read/
collectl.b-stageosd001-r19f29-prod.acme.symcpe.net collectl.v-stagemon-002-prod.abc.acme.net output.0.v-stagemon-001-prod.abc.acme.net
collectl.v-stagemon-001-prod.abc.acme.net historic_ops.out.b-stageosd001-r19f29-prod.abc.acme.net
`
Are there ways to now visualize this data.