task/restart: Restart task for testing daemon kill

The ceph daemons support being killed at a specific code point
with a config option.  In some cases, we want to test a kill point
only once for a given daemon run (such as replay that only occurs
during daemon startup).  This task allows running a script or executable
and (when the script sends a command to the task) restarting it with
a temporary config that has the appropriate kill point set.  Once
the daemon asserts and gets restarted, the original config is used.

Adds a specific restart_with_args() method to the DaemonState in the
ceph task.

Right now this task follows the workunit task closely, but uses stdout/stdin
to specify when to restart a daemon.

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