Add scenario test of instance snapshot and boot

This test is for snapshotting an instance and booting with it.
The following is the scenario outline:
 * boot a instance and create a timestamp file in it
 * snapshot the instance
 * boot a second instance with the snapshot
 * check the existence of the timestamp file in the second instance

Implements: blueprint add-instance-snapshot-scenario-tests
Change-Id: Ia20eaaae08af50d716c435e10e02d19d70d3adb5
Merge "Add scenario test of instance snapshot and boot"