Make API part of instance boot use new BDM format

This patch makes the API part of the instance boot process rely on the
new BDM format. It adds additional validations that are enabled due to
using the new format.

Since the new format will allow to specify devices that will be created
as images on the hypervisor (if the hypervisor enables it), in order
to prevent a DOS, a new config option 'max_local_block_devices' was
added. This option allows the operator to set a limit on the number of
destination_type='local' block devices a user can specify per VM.

This patch also refactors and simplifies how block devices are handled
during boot, and makes sure that validation errors are caught and
transformed to expected HTTP errors.

Change-Id: I62e0807fcff0a284ff72555d107cac1c9864e46d
blueprint: improve-block-device-handling
Change RPC to use new BDM format for instance boot

This patch makes rpc calls that are the result of booting an instance
propagate the new block device mapping format.

The way this is done is by adding a 'legacy_bdm' flag to the conductor
(task) API. Since both the scheduler and the compute service propagate
the block device mapping as part of the request_spec field, a flag named
legacy_bdm_in_spec to indicate that was introduced in their respective
APIs.

Currently block_device_mapping is not used by any of the in-tree filters
in the scheduler, so it is assumed that scheduler's boot functionality
has now transitioned to the new format.

This patch also bumps RPC versions of the task, scheduler and compute
manager classes.

Finally this patch propagates the legacy_bdm_in_spec through the
run_instance callback of the compute manager so that, in case of error -
the instance is re-scheduled with the proper flag set.

blueprint: improve-block-device-handling

Change-Id: I5f25ddd4d586dda91061f065c1796be726b0ede3
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: I444e91304248d648bb183fdf2c904812134f38de