provider/openstack: Allow subnets with no gateway
provider/openstack: Allow subnets with no gateway

This commit adds a no_gateway attribute. When set, the subnet will
not have a gateway. This is different than not specifying a
gateway_ip since that will cause a default gateway of .1 to be used.
This behavior mirrors the OpenStack Neutron command-line tool.

Fixes #6031

haxorof/terraform-provider-openstack,haxorof/terraform-provider-openstack,haxorof/terraform-provider-openstack