Add a generic post-prereq phase

This generic extra phase is handy for fixing up things like python
modules after pip is installed or other distro specific requirements.

Change-Id: I4a68b830cd98c6f93191dac1edd7de2a0381feaa
diff --git a/stack.sh b/stack.sh
index 497e8a1..93b7281 100755
--- a/stack.sh
+++ b/stack.sh
@@ -538,6 +538,12 @@
 
 install_rpc_backend
 
+# a place for distro-specific post-prereq workarounds
+if [[ -f $TOP_DIR/tools/${DISTRO}/post-prereq.sh ]]; then
+    echo_summary "Running ${DISTRO} extra prereq tasks"
+    source $TOP_DIR/tools/${DISTRO}/post-prereq.sh
+fi
+
 if is_service_enabled $DATABASE_BACKENDS; then
     install_database
 fi