Add Container#link_or_raise

  * this method is going to be used in staging task

Signed-off-by: Kris Kelly <kkelly@pivotallabs.com>
Use spawn/link instead of run_script in staging task

  * This will allow dea logging agent to connect to spawned job before it's completed

Signed-off-by: Kris Kelly <kkelly@pivotallabs.com>
Snapshot additional attributes for staging task

 * warden job id, container path, syslog drain urls
that DEA logging agent needs to connect to container's job

[#63400876]
Signed-off-by: Kris Kelly <kkelly@pivotallabs.com>