Add automatic retry to devstack install script

I don't know about anyone else, but whenever I kick off the
ds-build role on a fresh VM, I see a high rate of failure with
git clone operations ("git call failed" is the error), which kills
off stack.sh and results in a failed build.  This happens about 50%
of the time for me, so I modified the devstack build script to
automatically retry if it encounters this error.

Change-Id: If91577707ed76b1ec566af9189e30bbdcd81c37d

ds-build/roles/devstack-build/files/autostack.sh

@@ -39,9 +39,18 @@ su $unpriv_user -c $basedir/devstack/stack.sh 2>&1 &
 
 # Wait for stack.sh to complete by watching the log file for $donestring
 donestring='This is your host ip'
+# Sometimes, 'git clone' fails, and this can be retried
+retrystring='git call failed: \[git clone'
 
-while [ `tail -1 $log 2>/dev/null |grep -c "$donestring"` = 0 ]; do
-        sleep 5
+success=0
+while [ "$success" = 0 ]; do
+    if [ `tail -1 $log 2>/dev/null |grep -c "$donestring"` = 1 ]; then
+        success=1
+    elif [ `tail -2 $log 2>/dev/null |grep -c "$retrystring"` = 1 ]; then
+        pkill -f devstack/stack.sh
+        su $unpriv_user -c $basedir/devstack/stack.sh 2>&1 &
+    fi
+    sleep 10
 done
 
 # Kill off the now-idle stack.sh process