Add special serial smoke option

We want smoketests to be parallel by default (for grenade etc), but that won't work
for neutron, so add a special case. Since we don't want to break
anything making this switch, it has to be done in 3 steps:

1) this patch
2) make neutron use smoke-serial
3) change smoke to be parallel

Change-Id: I03ded120c0b496026e39f14f6715624a91ac7558
diff --git a/tox.ini b/tox.ini
index a3c781b..268f7e9 100644
--- a/tox.ini
+++ b/tox.ini
@@ -83,6 +83,14 @@
 commands =
    sh tools/pretty_tox_serial.sh '(?!.*\[.*\bslow\b.*\])((smoke)|(^tempest\.scenario)) {posargs}'
 
+[testenv:smoke-serial]
+sitepackages = True
+# This is still serial because neutron doesn't work with parallel. See:
+# https://bugs.launchpad.net/tempest/+bug/1216076 so the neutron smoke
+# job would fail if we moved it to parallel.
+commands =
+   sh tools/pretty_tox_serial.sh '(?!.*\[.*\bslow\b.*\])((smoke)|(^tempest\.scenario)) {posargs}'
+
 [testenv:coverage]
 sitepackages = True
 commands =