Add observer role to benchmark tool

This patch introduces a new observer role, which lists messages but
does not claim them. As part of this work the config options were
updated and the defaults adjusted to provide a better "kick the
tires" experience.

The default number of procs and workers is now hard-coded rather
than being based on number of available CPUs, since the number of
workers you may want to run is more dependent on the size of your
Zaqar deployment and your network bandwidth than it is on the
number of CPUs on the load generator.

Finally, the "-pc" suffix was removed from the command name. This
was included in this patch because it didn't seem significant
enough to split out.

Change-Id: I8a8190fb2cebc3489c78da4f6e1e7c51d8b97017
Merge "Add observer role to benchmark tool"