Add custom networks to Kitchen machines

Hi there,

One important part of some of our Chef recipes is the need to have our machines connected to additional Rackspace networks.

This patch allows users to specify a list of Rackspace networks they'd like their machine joined to, in addition to the mandatory PublicNet and ServiceNet networks - this behaviour is consistent with knife rackspace server create. I'm also lazy and can't be bothered typing all those 1s and 0s for the default Rackspace networks.

Please feel free to suggest improvements to the test - given how deep the final call to Fog::Compute.servers.bootstrap is, I found it a bit difficult to throw a Spy down there. Let me know if you can think of a better way!

Thanks,
Kane