Improve network robustness

This pull request is intended to address the rash of network issues that have been reported lately, including #102, #103, #104, #123, #125, and #126.

It makes a few changes that users may notice

* using a network named _vagrant-libvirt_ instead of _default_
* renaming the configuration option *default_network* to *management_network_name*
* adding the configuration option *management_network_address*

I've tried to break up the commits logically so you can understand what they each address, Overall the diff to create_networks looks messy because of renames and indentation changes.

I expect others will want to test this and/or provide feedback before it is merged. All that is appreciated.