Do not allow add multiple VLANs or subnets into shared network

CS allows to add multiple VLANs and subnets to a shared network, but it doesn't really work, for example , vm on second VLAN or subnet cannot get IP through DHCP
This patch will not allow to add mulitple VLANS or subnets into shared network

CLOUDSTACK-982:Do not allow add multiple VLANs or subnets into shared network