neutron_subnet does not handle empty dns servers, allocation pools and also specific host routes

neutron subnet was expanded with ability to to set and update parameters like
specifing dns servers, allocation pools and also specific host routes.
Sadly it introduced issue when calling neutron_subnet without specifying
values for these parameters.

This issue generates error:
Notice: /Stage[main]/Openstack::Provision/Neutron_network[public]/ensure: created
Error: Could not set 'present' on ensure: undefined method `[]' for nil:NilClass at 173:/var/tmp/packstack/9d9ca5a840ae48d684cb1537a0cf4142/modules/openstack/manifests/provision.pp
Error: Could not set 'present' on ensure: undefined method `[]' for nil:NilClass at 173:/var/tmp/packstack/9d9ca5a840ae48d684cb1537a0cf4142/modules/openstack/manifests/provision.pp
Wrapped exception:
undefined method `[]' for nil:NilClass
Error: /Stage[main]/Openstack::Provision/Neutron_subnet[public_subnet]/ensure: change from absent to present failed: Could not set 'present' on ensure: undefined method `[]' for nil:NilClass at 173:/var/tmp/packstack/9d9ca5a840ae48d684cb1537a0cf4142/modules/openstack/manifests/provision.pp
Notice: /Stage[main]/Openstack::Provision/Neutron_l3_ovs_bridge[br-ex]: Dependency Neutron_subnet[public_subnet] has failures: true
Warning: /Stage[main]/Openstack::Provision/Neutron_l3_ovs_bridge[br-ex]: Skipping because of failed dependencies
Notice: /Stage[main]/Openstack::Provision/Keystone_tenant[demo]/ensure: created
Notice: /Stage[main]/Openstack::Provision/Neutron_router[router1]: Dependency Neutron_subnet[public_subnet] has failures: true
Warning: /Stage[main]/Openstack::Provision/Neutron_router[router1]: Skipping because of failed dependencies
Notice: /Stage[main]/Openstack::Provision/Neutron_network[private]/ensure: created
Error: Could not set 'present' on ensure: undefined method `[]' for nil:NilClass at 183:/var/tmp/packstack/9d9ca5a840ae48d684cb1537a0cf4142/modules/openstack/manifests/provision.pp
Error: Could not set 'present' on ensure: undefined method `[]' for nil:NilClass at 183:/var/tmp/packstack/9d9ca5a840ae48d684cb1537a0cf4142/modules/openstack/manifests/provision.pp
Wrapped exception:
undefined method `[]' for nil:NilClass
Error: /Stage[main]/Openstack::Provision/Neutron_subnet[private_subnet]/ensure: change from absent to present failed: Could not set 'present' on ensure: undefined method `[]' for nil:NilClass at 183:/var/tmp/packstack/9d9ca5a840ae48d684cb1537a0cf4142/modules/openstack/manifests/provision.pp
Error: Could not prefetch neutron_router_interface provider 'neutron': undefined method `[]' for nil:NilClass
Notice: /Stage[main]/Openstack::Provision/Neutron_router_interface[router1:private_subnet]: Dependency Neutron_subnet[public_subnet] has failures: true
Notice: /Stage[main]/Openstack::Provision/Neutron_router_interface[router1:private_subnet]: Dependency Neutron_subnet[private_subnet] has failures: true
Warning: /Stage[main]/Openstack::Provision/Neutron_router_interface[router1:private_subnet]: Skipping because of failed dependencies