Make passing network id optional

Some cloud providers (like rackspace) automatically
add your instances to a network without having to
explicitly specify one.

This patch makes passing networks optional so that
you can just use the defaults in environmnets that
support them.
Merge pull request #4 from bodepd/net_optional

Make passing network id optional
Allow resource to set config drive

This patch allows roles to specify that
they require config drive.

Config drive is required on some clouds for
cloud-init to work (like rackspace)