Drive: allow to manually initialize a Nuxeo Drive instance

By first creating the configuration folder (~/.nuxeo-drive), the local folder (~/Nuxeo Drive) and an empty SQLite database (~/.nuxeo-drive.nxdrive.db), then populating the empty database with information such as:

• the Nuxeo server URL
• the proxy configuration

This can be useful for the deployment of Nuxeo Drive on a large set of desktops, allowing end users to work on a preconfigured instance, only needing to provide their credentials at first startup.