support Gradle as an alternative way to resolve dependencies

This changeset splits the actual resolution (and downloading) of the dependencies from the module management part, and introduces a second way of getting the JARs - using Gradle and Maven Central/JCenter. This allows packages relying on cmmnbuild-dep-manager to be deployed outside of CERN.

For the time being, either Gradle OR CBNG is used for all products. This is to avoid merging the resolved dependencies from two sources. This implies that packages which are used both within CERN and outside must provide a list of dependencies for either case (as CERN product names generally don't match the public ones)