satlab: mount rpm_keys in labservice

Mount rpm_keys at /creds/rpm in labservice so /creds/rpm/password.json
persists instead of getting wiped when the container is recreated.
Satlab runs SetRpmPowerState in labservice rather than drone.

BUG=b:580977561
TEST=docker-compose config and docker cp across container recreate
TAG=agy
CONV=8c4fa8f1-9c82-46c6-9f89-1e8487598473

Change-Id: I3f82ddc102297b6853da290b2f669f363b1ab22b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/satlab/+/8709255
Reviewed-by: Przemysław Filipek <filipek@google.com>
Commit-Queue: Jason Stanko <jstanko@google.com>
Tested-by: Jason Stanko <jstanko@google.com>