Do not clean dangling volumes that are not created by incubator-openwhisk-devtools without warning on make destroy

[motin]

Took this repo out for a spin. make quick-start, tested some things, make destroy. Behold, all my persistent docker volumes used in other development projects to persist data / database contents were deleted without a warning.

The line responsible for this is https://github.com/apache/incubator-openwhisk-devtools/blob/master/docker-compose/Makefile#L306

Either make sure not to "clean up" volumes that are not created by incubator-openwhisk-devtools or at least warn/ask before "cleaning" all these volumes.