Preconfigure the sample custom configuration template to replace plugins repository

People were used to directly put custom bundles in the deprecated plugins directory. That directory is kept for legacy compatibility but not recommended anymore.

Beside the few issues like random loading order of bundles, the main goal and rule to follow is that the server structure should never be manually edited. Changing the server content is the responsibility of the Nuxeo Runtime (including the Launcher and the Marketplace Package Manager). Users customize the server from outside with the nuxeo.conf file, the custom configuration templates and the Marketplace Packages. The partition between manually installed and automatically installed items is : the server structure versus the rest of the disk. That provides easy maintenance, history, versioning, confident team working and much more.
So, the plugins folder which was really handy when we hadn't those tools, is now a violation of that philosophy.

The custom configuration templates provide a similar solution, as much convenient as the plugins directory, but compliant with the above principle. The existing sample custom template can even be used (as suggested by the doc), though under that blessed directories hierarchy if it's a development server for which maintenance, lasting quality and team working are out of purpose.

nuxeo.conf: nuxeo.templates=default,/path/to/custom-bundles
custom-bundles
├── bundles
│   ├── a-custom-bundle.jar
│   └── another-bundle-0.1.jar
└── nuxeo.defaults: custom-bundles.target=nxserver

On production, it is recommended to place customization (nuxeo.conf and custom templates) under /etc/nuxeo/, data in /var/lib/nuxeo/, logs in /var/log/nuxeo/ and leave the $NUXEO_HOME/ directory untouched. That way, backup, restore and upgrade are really safe and easy.

=> Update the sample custom configuration template to match requirements for directly using it as a replacement to the plugins directory.