Adds os-volume_attachments 'volume_id' validation

If you POST to os-volume_attachments but give the name of the volume
instead of a UUID, it returns a 404 error; this is invalid HTTP.
404 means that the Request-URI has not been found. In fact, the
Request-URI has been found, but a child attribute has been incorrectly
specified. Some other error could be used.

This fixes the problem by returning 'BadRequest' in place of 'NotFound'.
The 'volumeId' is checked to be a valid UUID string before moving
further.

Fixes bug #1062494

Change-Id: Icc5dbc7ac94051514709997457cafb16e870bea9
Merge "Adds os-volume_attachments 'volume_id' validation"