Make screen sleep time configurable

the sleep 3 in screen_it was added to make devstack pass in the gate
with exceptionally slow test cloud nodes. In the gate we now bypass
the screen path entirely. However the sleep 3 remains and can add a
couple minutes delay into local development runs.

We're not sure yet how low this can safely be tuned, so step 1 is to
make it configurable, then get devstack team members to try various
options to see what works.

Change-Id: I0e6476176fc8589efc4e40e78c2231f704d14e45
diff --git a/functions-common b/functions-common
index f442211..f8543c1 100644
--- a/functions-common
+++ b/functions-common
@@ -1256,8 +1256,13 @@
 
     # sleep to allow bash to be ready to be send the command - we are
     # creating a new window in screen and then sends characters, so if
-    # bash isn't running by the time we send the command, nothing happens
-    sleep 3
+    # bash isn't running by the time we send the command, nothing
+    # happens.  This sleep was added originally to handle gate runs
+    # where we needed this to be at least 3 seconds to pass
+    # consistently on slow clouds. Now this is configurable so that we
+    # can determine a reasonable value for the local case which should
+    # be much smaller.
+    sleep ${SCREEN_SLEEP:-3}
 
     NL=`echo -ne '\015'`
     # This fun command does the following: