vagrant: Working flat provider network with neutron/ml2.

Pulled out existing networking and set up a single flat provider network.

This means that instances can be reached from the default network namespace,
and can even be pinged by the host machine that's running vagrant.

Floating IPs and tenant vlans will come in future PRs.
Do not set flat network options by default in ml2
updated based on @j2sol suggestions
Merge pull request #635 from blueboxgroup/vagrant_flat_network

vagrant: Working flat provider network with neutron/ml2.