Support for user groups and assigning roles to them apart from derictly to users

[tassosgeo]

Is your feature request related to a problem? Please describe.
Currently superset only support user roles and not also user groups.
This will be inline with most openid authentication providers so that groups and roles can propagate from them to superset.
Currently both of them have to be mapped as roles in superset.

Describe the solution you'd like
To be able to create user groups and assign roles to these groups, and not only assign roles to users directly