Make the launcher feed the SDK on Marketplace package install

When installing a Marketplace package, that MP deploys properties files which list GAV/libraries mapping, to be used by the SDK.
Usually, the "resolution" of those properties (meaning downloading the JARs corresponding to the listed GAV) is done at assembly time. This is not relevant when installing a MP. So make the Launcher able to download those JARs when installing a MP into a SDK server.

For that purpose, the Launcher should use a default Maven repository URL (i.e. https://maven.nuxeo.org/) but we'll also need to specify a custom URL for a given set of JARs. That could be managed with another file. For instance, "custom.m2repo" would provide a specific repository for resolving GAV from "artifacts-custom.properties" and "test-artifacts-custom.properties"