Support Volume based for amphorae instance

In many openstack deployment are using Volume based for their instance. So if amphorae instance boot from localdisk is not good for this case. We want to have an option to create volume and boot from this volume. The pros is in the case of failover, we can boot an other instance from this volume. It's fast.

I have some commits for this specs but I don't know how to contribute to this project.

Here is commit diff: https://github.com/greatbn/octavia/commits/master