need special handling for reverse_dns_zone_group container attribute (just like reverse_dns_profile)

[miesi]

To minimize administrative efforts with automatically appearing reverse zones when subnets are added to pools a container attribute reverse_dns_zone_group shall be introduced to automatically put reverse zones in the correct zone-group.

# <assume zone-group public global and zone-profile public-rev exists>
$ ndcli create container 1.0.0.0/8 reverse_dns_profile:public-rev reverse_dns_zone_group:public-global
$ ndcli create pool p
$ ndcli modify pool p add subnet 1.0.0.0/24
$ ndcli list zone-group public-global
zone                                                     view
0.0.1.in-addr.arpa                                 default