warden: don't destroy containers on startup

We're using the snapshot feature again, so this is counterproductive.

Old containers should be destroyed via the grace period if no one's
actually talking to them.
update drain script to add snapshotting

If the job is shutting down, execute the deterministic evacuation
policy.

Otherwise, send USR1 once, and wait for all staging tasks to complete.
After that, send USR2 to Warden, and kill -9 the DEA. No evacuation
is performed. Both jobs will come up and repopulate their state via
their respective snapshot loading mechanisms.

[finishes #57361124]

Signed-off-by: Onsi Fakhouri <onsi@pivotallabs.com>