It is common for Neutron deployment's policy to forbid GETs to the
public subnet, only allowing GETs for the public net. Since the only
required field of those two for creating a FIP is the public net, let's
change public net to be the only required config option and have the
subnet stick around as optional.
Change-Id: I31c3c51ad2dc12f8f560cbab01c86d04aabb754e
Closes-Bug: 1749921
Signed-off-by: Antoni Segura Puimedon <antonisp@celebdor.com>