scap: post runtime stats to statsd / graphite

* Add a simple statsd metric reporter class. It should be factored out into a
  separate file as soon as we start turning this in a proper Python module.
* Report scap runs to Graphite by incrementing a scap counter and by reporting
  the time scap took to run.

Change-Id: Id28031b5ec0451aa030be73ccbc6c9471517a16c
scap: invoke dsh via subprocess.call rather than .check_call

dsh frequently fails for mundane reasons -- for example, because a host was
taken out of circulation for repairs but not removed from dsh group. scap just
keeps going, so the Python re-implementation should too, as pathological as
that may seem.

Change-Id: I247995f2bd2c2431063e3ec071c64066d98c77dd