Allow nonsuperuser roles to manange disk qouta

[reshke]

The purpose of these patches is to allow non-superuser roles to managed disk qouta in greeplum cluster. This is helpfully for cloud providers which can serve managed greenplum solution. This is undesirable to grant superuser to some customer roles, because superusers are able to perform almost anything on cluster VMs. The solution in to create some 'special' role (diskquota_admin) with needed permission and grant it to customer-side role