Fix bug 666 - Neutron Subnet Creation Error

When a neutron subnet is created on the Northbound API, containing the
new "ipv6_address_mode" or "ipv6_ra_mode" fields with "null" values
Jersey will fail to deserialise the object correctly and throw an HTTP
500 error with a NullPointerException.

This commit adds the above fields to the NeutronSubnet object and
permits these fields to be nillable.

Change-Id: I225581b57a1698a5e5c17f91b117fba556b70976
Signed-off-by: Dave Tucker <dave.j.tucker@hp.com>
Signed-off-by: Dave Tucker <djt@redhat.com>
Merge "Fix bug 666 - Neutron Subnet Creation Error" into stable/hydrogen