Bugfix argument passed to docker run.

Change -volumes-from to --volumes-from.
Refactor to remove big duplicated chunk of code.

Checked for difference between the two blocks first - the only
difference was the apparent typo fixed in ca30f761ace.
Bugfix check of installed tag name.

Don't mistake e.g. installed tag '0.11' for requested tag '0.1'.
Add checksum verification support.

NOTE: This is a bit of a hack, and part of it (the shellscript
/usr/local/bin/verify_docker_image) is NOT distributed with this
puppet module!