Enable optional Python 3 support

Add USE_PYTHON3 and PYTHON3_VERSION variables to allow services to use
python 3 if they indicate support in their python package metadata.

Tested in Heat here -> I837c2fba682ab430d50e9f43913f2fed20325a7a.
Project config change to add a dedicated job to Heat is here -> I0837e62d6ccc66397a5e409f0961edd4be31f467

Change-Id: I079e18b58b214bf8362945c253d6d894ca8b1a6b
diff --git a/tools/install_prereqs.sh b/tools/install_prereqs.sh
index 38452cd..031f8a8 100755
--- a/tools/install_prereqs.sh
+++ b/tools/install_prereqs.sh
@@ -81,6 +81,9 @@
     fi
 fi
 
+if python3_enabled; then
+    install_python3
+fi
 
 # Mark end of run
 # ---------------