-Only require one iteration to build group->member hash. This speeds up:
-nodeadd when a lot of nodes already exist
-any noderange with a lot of 'atoms' that aren't nodes
git-svn-id: https://svn.code.sf.net/p/xcat/code/xcat-core/trunk@4377 8638fb3e-16cb-4fca-ae20-7b5d299a9bcd