Stop doing any magic cloning of neutron during CI

Hopefully this will allow the Zuul "Depends-On" thing to work better.
Adopting the solution developed by Doug Wiegley for LBaaS:
c61f920f2a54fd829ee37484b3743ab77b909847 and then shamelessly stolen
from armando-migliaccio here: If358edd5cf34fd63b508dc089615bf19d59ee334

Change-Id: I69c53bb2098b30927726e03c6c492a2fb58d4cc2

tools/tox_install.sh

@@ -0,0 +1,42 @@
+#!/bin/sh
+
+# Many of neutron's repos suffer from the problem of depending on neutron,
+# but it not existing on pypi.
+
+# This wrapper for tox's package installer will use the existing package
+# if it exists, else use zuul-cloner if that program exists, else grab it
+# from neutron master via a hard-coded URL. That last case should only
+# happen with devs running unit tests locally.
+
+# From the tox.ini config page:
+# install_command=ARGV
+# default:
+# pip install {opts} {packages}
+
+ZUUL_CLONER=/usr/zuul-env/bin/zuul-cloner
+neutron_installed=$(echo "import neutron" | python 2>/dev/null ; echo $?)
+
+set -ex
+
+cwd=$(/bin/pwd)
+
+if [ $neutron_installed -eq 0 ]; then
+    echo "ALREADY INSTALLED" > /tmp/tox_install.txt
+    echo "Neutron already installed; using existing package"
+elif [ -x "$ZUUL_CLONER" ]; then
+    echo "ZUUL CLONER" > /tmp/tox_install.txt
+    cd /tmp
+    $ZUUL_CLONER --cache-dir \
+        /opt/git \
+        git://git.openstack.org \
+        openstack/neutron
+    cd openstack/neutron
+    pip install -e .
+    cd "$cwd"
+else
+    echo "PIP HARDCODE" > /tmp/tox_install.txt
+    pip install -U -egit+https://git.openstack.org/openstack/neutron#egg=neutron
+fi
+
+pip install -U $*
+exit $?

tox.ini

@@ -9,9 +9,8 @@ skipsdist = True
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
 usedevelop = True
-install_command = pip install -U {opts} {packages}
-deps = -egit+https://git.openstack.org/openstack/neutron#egg=neutron
-       -r{toxinidir}/requirements.txt
+install_command = {toxinidir}/tools/tox_install.sh {opts} {packages}
+deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 whitelist_externals = sh
 commands =