launch-node.py: request flavors/images by ID or name

It is deemed useful by some to pass IDs rather than names to
launch-node.py for flavors and images; mostly due to the fact that
names are not unique, and the "first-matched" behavior on names
means that it can end up doing the wrong thing in certain cases.

Change-Id: Ief89174fedac7921f85bbfe3a456059ecc0f19bf
Merge "launch-node.py: request flavors/images by ID or name"