Fix neutron-tempest-plugin tox pep8 setup

The plugin pep8 setup was failing due to trying to use upper-constraints
with the pip install -e ., with a version constraint error. Grab
the setup commands from the neutron tox.ini to fix this.

Change-Id: Ic7b6e80213aafac6df7b0a8b044fe45f60870483
diff --git a/tox.ini b/tox.ini
index 82a473c..9941e12 100644
--- a/tox.ini
+++ b/tox.ini
@@ -5,14 +5,17 @@
 
 [testenv]
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
    OS_LOG_CAPTURE={env:OS_LOG_CAPTURE:true}
    OS_STDOUT_CAPTURE={env:OS_STDOUT_CAPTURE:true}
    OS_STDERR_CAPTURE={env:OS_STDERR_CAPTURE:true}
-deps = -r{toxinidir}/test-requirements.txt
+install_command =
+  pip install {opts} {packages}
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]