Source functions before stackrc

The enable_service() and disable_service() functions in
https://review.openstack.org/9407 require the functions
file be sourced before stackrc.

* exercise.sh
* samples/local.sh (this will require manual addition to the user's
  local.sh if they based it on the sample)
* tools/build_bm.sh

Change-Id: I1bed687867e870bef5748289d712376435a776af
diff --git a/exercise.sh b/exercise.sh
index 15f264f..a0349ce 100755
--- a/exercise.sh
+++ b/exercise.sh
@@ -5,6 +5,9 @@
 # Keep track of the current devstack directory.
 TOP_DIR=$(cd $(dirname "$0") && pwd)
 
+# Import common functions
+source $TOP_DIR/functions
+
 # Load local configuration
 source $TOP_DIR/stackrc