Avoid selecting controller nodes for storage

If no nodes was specifically selected as storage node,
prefer the non-controller nodes for storage.

Helps with https://bugzilla.novell.com/show_bug.cgi?id=882808

(cherry picked from commit 18ff4cc8cd42f9776d48ef18e716aa80cc114524)
Merge pull request #99 from dirkmueller/release/stoney/master

Avoid selecting controller nodes for storage