Fix Vagrantfile to launch multiple nodes

Got the fix from https://github.com/bunchc/Couch_to_OpenStack/pull/10 to branch, separated it from my private adjustments. Shall be easy to pull now.

====
I tried to create multiple compute nodes and it didn't. Found a bug (dah), here is a one line fix.

Regards, Dmitri.

PS. Thanks for the great series.