Install prettytable>=0.7 to satisfy pip 6/PEP 440

Also use sudo -H with pip so that it doesn't create a ~stack/.cache
other things can't write to as the stack user later.

Change-Id: I2134c7d8f58f8b83f33150c9ed86d87f8ccba2f3
diff --git a/functions-common b/functions-common
index 94ab347..40a0d2b 100644
--- a/functions-common
+++ b/functions-common
@@ -1578,7 +1578,7 @@
         local sudo_pip="env"
     else
         local cmd_pip=$(get_pip_command)
-        local sudo_pip="sudo"
+        local sudo_pip="sudo -H"
     fi
 
     $xtrace