There are two default resource groups, default_group and admin_group,
roles will be assigned to one of them depending on they are superuser or
not unless a group is explicitly specified. Only superuser can be
assigned to the admin_group.

    -- create a role r1 and assign it to resgroup g1
    CREATE ROLE r1 RESOURCE GROUP g1;
Signed-off-by: NNing Yu <nyu@pivotal.io>