Adds support for static IPs and interface config

This enables to plugin to assign static IPs to private networks (taken
from the standard high-level network configuration), and also to
configure the corresponding interfaces (with the exception of the main
one) in the VMs.
Allocate IPs automatically

This makes the plugin allocate an IP automatically if `floating_ip` is
set to :auto and `floating_ip_pool` is configured.  Addresses allocated
in this manner are then released when the server is destroyed.

If `floating_ip_pool` is not set, the behavior of the plugin doesn't
change.
Merge pull request #87 from arbrandes/master

Static IP assignment, private network interface configuration, and floating IP address allocation