Import NetworkTopology from the correct module.

Fixes the following exception:

    $ ./bin/evilgenius
    Traceback (most recent call last):
      File "./bin/evilgenius", line 12, in <module>
        from evilgenius.vagrant import VagrantController, NetworkTopology, VagrantBox
    ImportError: cannot import name NetworkTopology
Merge pull request #2 from sburnett/master

Import NetworkTopology from the correct module.