fix: limit neutron vpnaas ref (#1458)

The cause for this is due to projects are around release cycle that makes the dependency conflict openstack/neutron-vpnaas@f2fcd45
this is currently blocking our neutron image build jobs in #1428
diff --git a/images/neutron/Dockerfile b/images/neutron/Dockerfile
index 0d91753..d6b3a36 100644
--- a/images/neutron/Dockerfile
+++ b/images/neutron/Dockerfile
@@ -7,7 +7,8 @@
 ARG NEUTRON_GIT_REF=019294c71d94b788c14b23dc1da3c21f51bcdb0b
 ADD --keep-git-dir=true https://opendev.org/openstack/neutron.git#${NEUTRON_GIT_REF} /src/neutron
 RUN git -C /src/neutron fetch --unshallow
-ADD --keep-git-dir=true https://opendev.org/openstack/neutron-vpnaas.git#master /src/neutron-vpnaas
+ARG NEUTRON_VPNAAS_GIT_REF=7bc6d94305d34269d7522a9850c22aa42b50cdab
+ADD --keep-git-dir=true https://opendev.org/openstack/neutron-vpnaas.git#${NEUTRON_VPNAAS_GIT_REF} /src/neutron-vpnaas
 RUN git -C /src/neutron-vpnaas fetch --unshallow
 RUN --mount=type=cache,mode=0755,target=/root/.cache/pip,sharing=private <<EOF bash -xe
 pip3 install \