[ADD] http: addons preload opt-out for socket activation

Before this patch, early connections made to a 11.0 Odoo server running
on Python 3 and deployed in threaded mode with socket activation would
generate invalid registries.
With this patch it is now possible, when this deployment mode is used,
to opt-out addons preload by setting the following environment variable:

    ODOO_PRELOAD_ADDONS=no

Note: this environment variable is only available for v11.0 as later
versions does not preload anymore (cf: 1a39c9b)