Add openstack support

Public IP addresses of *already started* virtual machines will be added to the hosts file.
This is different from the virtualbox support since it is impossible to previously know what ip addresses will be attributed by OpenStack.

In other words, you will be required to do a "vagrant provision" pass after bringing up all the machines if you require every machine to know each other. This issue is not being addressed by this commit.

Combining OpenStack machines with VirtualBox's private network in the same multi-vm file is unsupported and will give unexpected results.

Reviewed-by: Mathieu Mitchell <mat128@gmail.com>