set bdm['volume_id'] to None rather than delete it

bdm is a sqlalchemy model object, it doesn't make sense to delete
an attribute, so just set it to None

Fix bug 1166160

Change-Id: I41017af602f0c8369d442b4a6936e1a829d2f9a9
Send a instance create error notification

Send a 'create.error' notification if instance creation (run_instance)
fails.

This will make it easier for external notification consuming systems to
learn when instance build attempts fail within the nova-compute layer.

blueprint create-error-notification

Change-Id: I62d7c9c80c51241bf124509af7cdd8484d9ea2d3
Merge "Send a instance create error notification"
Merge "set bdm['volume_id'] to None rather than delete it"