Fix missing serial option of tox full-serial target

This commit fixes the missing 'serial' option of tox 'full-serial'
target.

Change-Id: If33d8ad6135752fb31228c02810b36a3b9825018
diff --git a/tox.ini b/tox.ini
index 05fa326..678bbd0 100644
--- a/tox.ini
+++ b/tox.ini
@@ -88,7 +88,7 @@
 # See the testrepository bug: https://bugs.launchpad.net/testrepository/+bug/1208610
 commands =
     find . -type f -name "*.pyc" -delete
-    tempest run --regex '(?!.*\[.*\bslow\b.*\])(^tempest\.(api|scenario))' {posargs}
+    tempest run --serial --regex '(?!.*\[.*\bslow\b.*\])(^tempest\.(api|scenario))' {posargs}
 
 [testenv:smoke]
 envdir = .tox/tempest