Enhancement: Add option to skip generation of demo users and groups

We've added a new environment variable to decide whether we should generate the demo users and groups or not. 
This environment variable is set to `true` by default, so the demo users and groups will get generated by default as long as oCIS is in its "technical preview" stage.

In any case, there are still some users and groups automatically generated: for users: Reva IOP, Kopano IDP, admin; for groups: sysusers and users.

https://github.com/owncloud/ocis/pull/2495
