Add a varnish service check.

The service check uses the `varnishadm debug.health` command to
get the health of available backends. One check per backend will
be submitted, tagged by the backend name.

The BIG question here is how we will let users enable this because
the varnishadm command requires access to the secret key which has
root permissions by default (and is owned by root).
Merge pull request #1213 from DataDog/conor/varnish-service-check

Add a varnish service check.