prefer floating ips if available

At Deutsche Telekom we have an openstack cluster that requires vpn connections. On the one hand we need floating ips for kitchen tests from local machines. On the other hand ci tests do not need this. 

This pull requests raises the questions about the ip selection process. The current ip discover process detects all ips but the user has no possibility to select a specific order. Currently the pull request fixes the order to floating, public, private.

I also like the idea of having an additional driver config like `ssh_network: floating`. In such case we just distingish this case in `def get_ip(server)`. What do you think?