BREAKING CHANGE: SqlDatabaseRole: Should only handle database roles and not create database users

[bozho]

xSQLServerDatabaseRole resource currently creates a DB user for an SQL login and assigns already existing roles to the user.

The related issue (#846) proposes renaming the resource to xSQLServerDatabaseUser.

We should then implement a new resource xSQLServerDatabaseRole, used to handle DB roles.

[johlju]

I changed the title on this issue to better reflect the change that should be done. Correct me if I'm wrong.
Also see issue #846.

I suggest we change this resource to use the same/similar schema.mof as xSQLServerRole.

[johlju]

@bozho Could you propose a schema.mof for this as well?