fix devstack local.conf tweaks to tempest and neutron
fix devstack local.conf tweaks to tempest and neutron

The local.conf tweaks to tempest and neutron used by qa_devstack.sh
never worked, because the wrong kind of escaping was used.  $$
expanded to whatever pid number the code happened to be running under,
resulting in local.conf containing something like

    [[test-config|9471TEMPEST_CONFIG]]
    [compute]
    min_compute_nodes = 1

    [[post-config|9471NEUTRON_CONF]]
    [DEFAULT]
    global_physnet_mtu = 1450

This was because the heredoc was done as <<EOF therefore subject to
normal shell double-quotes interpolation.  However this interpolation
is required in order for the

    SERVICE_IP_VERSION=$SVC_IP_VERSION

line in local.conf to expand correctly.

So instead escape the $ correctly with backslash.

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