Process image BDM earlier to avoid duplicates

If the block device mapping from the image properties are not handled at
the same time as the ones defined from the API, we might make the false
assumption that the instance has no root and failing or, in the case
we're booting from an image that only defines a BDM and has no disk,
creating a local disk BDM as root and ending up with two root devices
which is forbidden.

This patch moves the handling of the image block device mappings to the
same method were we check the ones provided throught the API. This
allows nova to decide wether the instance needs a local disk from an
image if no root device is defined in any of the block device mappings.

Change-Id: Ide95357895ab4dd1338ab5ee3ec25294af1d010b
Closes-Bug: #1246327
Merge "Process image BDM earlier to avoid duplicates"