labs: add snapshot_cycle command

This changeset adds the new command, snapshot_cycle. It can be used to
replace four lines that often go together. The command handles a full
cycle needed for a snapshot: queueing shutdown.sh, booting the VM,
waiting for it to shutdown, and taking a snapshot.

Change-Id: I13276088bd44326f475bf7477ad2ed27db2dbbe9
Merge "labs: add snapshot_cycle command"