labs: verify MD5 checksum for Linux ISO image
labs: verify MD5 checksum for Linux ISO image

Verify the MD5 checksum of the Linux distribution's ISO image before
trying to use it. Image corruption happens, and the resulting problems
are not always easy to diagnose.

Change-Id: I92852a28392002d4fcd64898453086d5920d0ef1

openstack/training-labs,dguitarbite/OpenStack-Labs,openstack/training-labs,dguitarbite/OpenStack-Labs,openstack/training-labs,dguitarbite/OpenStack-Labs