Refactor how bdm is handled when booting

This patch refactors how block devices are handled when booting VMs.
Also it introduces new block device mapping format in the run_instance
manager method of the compute service.

The refactoring is done by creating a simple hierarchy of
DriverBlockDevice classes that are meant to:

1) Provide a common place to keep information about the data that is
kept for each block device type

2) Encapsulate the logic of "attaching" the device from the compute
service point of view (driver will still need to do the actual work).

Currently we have special subclasses for ephemeral, swap, volume and
snapshot devices.

These classes also make it possible to easily provide
the drivers that will require old block device mapping format with it by
overriding the legacy() method.

Another advantage that this refactoring brings is that adding additional
block device types, like Image block devices (coming soon in a subsequent
patch) that can have destination type of either a hypervisor image or a
volume will be just a matter of extending the base class and redefining
attach to do the work of creating the volume if needed.

blueprint: improve-block-device-handling

Change-Id: I5b9c3e2d959c602fa22f49db681da918ae0adcea
Allow empty device_name

Change-Id: I8fa9e647d525aa0837926ab066ab94dc10f642f6