[1878] add run_lettuce.sh script, and check in terrain

tests/lettuce/README.tutorial

@@ -54,7 +54,7 @@ The one scenario we have has no steps, so if we run it we should
 see something like:
 
 -- output
-> lettuce
+> ./run_lettuce.sh
 Feature: showing off BIND 10
   This is to show BIND 10 running and that it answer queries
 
@@ -95,7 +95,7 @@ it to be started before we continue.
 And let's run the tests again.
 
 --
-> lettuce
+> ./run_lettuce.sh
 
 Feature: showing off BIND 10
   This is to show BIND 10 running and that it answer queries

tests/lettuce/run_lettuce.sh

@@ -1,7 +1,7 @@
 if [ "$1" = "-I" ]; then
     shift
     echo "$@"
-    LETTUCE_SETUP_COMPLETED=1 lettuce $@
+    LETTUCE_SETUP_COMPLETED=1 exec lettuce $@
 else
     source ./setup_intree_bind10.sh
     exec lettuce $@