add better OS detection for BDM

Referencing #2438 - based on volume best practices detect the OS using password data.

This works against AWS EC2 but not against Eucalyptus (found a bug which I need to file).  

I'd be inclined to merge regardless to see if this can be fixed in Euca 3.3.  For Eucalyptus users it means they must override the block device mapping by ensuring they specify /dev/XXX.  If we can't patch this in Euca, then I'll introduce a workaround in the module this week.