Add validations to Compute API for block device mappings

[Rohit Karajgi]

While booting an instance with block device mappings, certain scenarios need to be
taken care of in the Compute API so that Invalid user inputs are avoided and instances do not
land up in inconsistent states.
We need to add validations in the API to ensure the request body contain correct combinations
of num_instances and block_device_mappings.

For example:
1. Boot multiple instances using a single block device mapping.
Current behavior: Only one instance boots successfully with the volume attached to it.
The other instances fail block device mapping and land up in ERROR state as the volume is "in-use".
User needs to cleanup ERROR instances.
API Response: 202 Accepted

Expected behavior: API should not allow multiple instances to boot from same volume.

2. Boot multiple instances from a list of block device mappings (snapshots or volumes):
Current behavior: Only one instance boots successfully. Only the second volume in list gets attached to the instance (first one) on vda.
The other instances fail block device mapping and land up in ERROR state as the volume is "in-use".
No 1:1 mapping between volume list and instance list
User needs to cleanup ERROR instances.
API Response: 202 Accepted
Expected behavior: API should not allow multiple instances to boot from a list of BDMs

This was discussed on the mailing list and a need for appropriate API level validations was accepted.
http://lists.openstack.org/pipermail/openstack-dev/2012-December/004149.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/18716

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/18716
Committed: http://github.com/openstack/nova/commit/e34d5d9e3d416fc40f147a5914f7dceee26f5ba4
Submitter: Jenkins
Branch: master

commit e34d5d9e3d416fc40f147a5914f7dceee26f5ba4
Author: Rohit Karajgi <email address hidden>
Date: Fri Dec 28 04:43:14 2012 -0800

    Add Compute API validations for block device map

    This patch adds validations to return 400 BadRequest to
    Create server(s) with block device mappings where appropriate
    and related unit tests

    Fixes bug 1094244

    Change-Id: I778d3ed4b0871561774594e950ce98b6a1ecfcb9