Show a list of all engine versions in build-info output

The build-info output currently shows the version of whichever engine happens to respond at the time. This makes it impossible to determine with complete confidence the version of every Heat engine running, which is useful in validating that a Heat deployment was successful.

Instead, build-info should return a list of engine ID-to-version mappings, so there is no doubt about the version of Heat running on every engine.