Add posargs to testr-full tox job

This commit adds posargs to to the testr-full tox job. This enables
passing extra arguments to testr when running the tox job.

Change-Id: I349e1aa21ce4e5e93ffd7e2c9667f2cbbec2a413
diff --git a/tox.ini b/tox.ini
index eb1ef4b..93a53ac 100644
--- a/tox.ini
+++ b/tox.ini
@@ -30,7 +30,7 @@
 sitepackages = True
 setenv = VIRTUAL_ENV={envdir}
 commands =
-  sh tools/pretty_tox.sh 'tempest.api tempest.scenario tempest.thirdparty tempest.cli'
+  sh tools/pretty_tox.sh 'tempest.api tempest.scenario tempest.thirdparty tempest.cli {posargs}'
 
 [testenv:smoke]
 sitepackages = True