Split an instance's path into several individual paths

In current nova version, all the information and files of an instance share the same path. The libvirt.xml, ephemeral disks, swap disk and config drive are all stored in the same directory.

This design seems a bit inflexible and brings some annoying problems.

For example, we can't put swap disk on local while put config drive on shared storage. And since live-migration with config drive or CDROM on local storage still not possible, we have to put all swap disk, ephemeral disk and config drive on shared storage. This may bring large stress on shared storage and affects the I/O performance.

Therefore, this blueprint proposes to split an instance's path into several individual paths and make it configurable.

For example, we can have
1) instances/configuration/(instance-id)/
2)instances/ config_drive/(instance-id)/
3) instances/ swap/(instance-id)/
4) instances/ ephemeral/(instance-id)/
5) instances/ cdrom/(instance-id)/