Adds floating-ips support ("os-floating-ips" extension for nova)

This pull adds support for the os-floating-ips extension to nova. This comes from a request to add support for 'floating IPs' and 'security-groups' to the Deltacloud OpenStack driver - the original request is tracked in [ASF JIRA](https://issues.apache.org/jira/browse/DTACLOUD-563). 

Support for security-groups already exsists (see README). Once I get a review for this and merge then I can start to work on the Deltacloud driver side,

thanks, marios