Fix to avoid undefined variables in initial sanity checks

This commit moves check to see if screen is already running after reading
stackrc, and changes messages in check if root is running DevStack.
STACK_USER is defined in stackrc, and SCREEN_NAME can be replaced with
"a non-root account".

Change-Id: I022ed1099169a3549b7e31883abe33d6298f6fa5
Closes-Bug: #1362091
Merge "Fix to avoid undefined variables in initial sanity checks"