task/pexec: More fixes for all case, exec on hosts

We don't want to do an exec per role, but per-host.  We
were already doing an exec per host, but the names were confusing.
This fixes the names up and removes the role parameters.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
task/pexec: Add barrier capability

This patch adds the ability to barrier between
parallel exec tasks so that all tasks will perform
the following step (after the barrier) at the same
time.

Signed-off-by: Sam Lang <sam.lang@inktank.com>