Iterate over nodes rather than networks in project_apply

When iterating over networks, unattached ports won't be applied.  This created
a bug where nodes were never actually removed from networks.  Iterating over
the nodes instead ensures that every relevant port is configured.

This fixes #200
dell: Document surprising success of detaching port

While fixing #200, ryabouza and I tried fixing the problem here, figuring that
sending 'None' instead of a VLAN ID was clearly incorrect.  But it's actually
fine!
dell: Make previous magic explicit instead
Merge pull request #201 from gsilvis/remove-port

Remove port from vlan correctly