Put devstack-version info into separate file

As a follow on to I4c269a7f3d63ee9a976e7c3636fc3e5e8dab9ae3; the
quoting gets tricky when putting arbitrary command-substitution
strings into saved echo-able strings.  As they say, "the only winning
move is not to play" :)

An alternative proposal is to not write this into a script but just
dump info into a file.  To my mind, this has several advantages --
avoid getting involved in quoting, not dropping a script into the
global environment -- it's just as easy to "cat" -- and the plain-text
file can be collected as an artifact during log collection (also moved
git commit line to separate line for easier parsing during log search,
etc).

Change-Id: Ic7391dd087657c0daf74046e4a052c53f4eb6e1a
diff --git a/stack.sh b/stack.sh
index fd18651..c453b91 100755
--- a/stack.sh
+++ b/stack.sh
@@ -216,7 +216,7 @@
 fi
 source $TOP_DIR/stackrc
 
-# this installs a devstack-version script to make it easy to report the version back
+# write /etc/devstack-version
 write_devstack_version
 
 # Warn users who aren't on an explicitly supported distro, but allow them to
@@ -1538,8 +1538,8 @@
     echo
 fi
 
-# devstack version
-devstack-version
+# Useful info on current state
+cat /etc/devstack-version
 echo
 
 # Indicate how long this took to run (bash maintained variable ``SECONDS``)