Provide tox entry for running slow heat tests

This will ultimately run parallel but get working with serial.

Change-Id: Ia8f51b56d5b0dec553639c610e68f61831c93c12
diff --git a/tox.ini b/tox.ini
index 471fecb..ea27b92 100644
--- a/tox.ini
+++ b/tox.ini
@@ -27,6 +27,13 @@
 commands =
   sh tools/pretty_tox.sh '(?!.*\[.*\bslow\b.*\])(^tempest\.(api|scenario|thirdparty|cli)) {posargs}'
 
+[testenv:heat-slow]
+sitepackages = True
+setenv = VIRTUAL_ENV={envdir}
+# The regex below is used to select heat api/scenario tests tagged as slow.
+commands =
+  sh tools/pretty_tox_serial.sh '(?=.*\[.*\bslow\b.*\])(^tempest\.(api|scenario)\.orchestration) {posargs}'
+
 [testenv:py26-full]
 sitepackages = True
 setenv = VIRTUAL_ENV={envdir}