Allow "installers" to specify pages that should be skipped in the wizard

Some settings (paths, database) can be set up by the installer (Windows setup.exe, debian package).
In that case, we don't want to show them in the wizard as modifying them will just break the application.

Suggested implementation:

• the installer sets "nuxeo.wizard.skip=pageName1,pageName2"
• the wizard reads that property and acts on it