REVIEW: [NX-389] consume karaf features

This switches nexus-karaf-template over to use Karaf's standard approach for assembling custom distributions and uses the generated features to install the Nexus base + OSS edition. Plugins are installed as features and unpacked under plugin-repository (removal of the plugin-repository is covered by the follow-up story to remove  the nexus-plugin packaging type, as plugins will then no longer be shipped as zips that require unpacking).