Fix the class assignment code in the master config script

Prior to this commit, there were a few bugs in the code
that was assigning classes to a node.  First, the assignment
was happening in a loop over the desired classes... so if there
were no desired classes, then no assignment would occur at all.

Second, we were using the `classes` rake task inside the loop
to assign a single class.  Since that task overwrites the
entire list of classes for a node, the end result would be
that the node would have exactly one class assigned to it, and it
would be whichever class appeared last in the list of desired
classes.
Merge pull request #8 from cprice-puppet/bug/master/PE-520-node-class-assignments

Fix the class assignment code in the master config script