Raise exception when Glance metadata not found.

It'd be better to raise exception when trying to copy Glance metadata
from source (volume/snapshot) to destination (volume/snapshot) rather
than silent failure, which is exactly the reason there's unspotted error
inside glance_meta unittest but it was able to pass.
With this fix, one should _not_ directly call glance_metadata_copy()
without looking at source's bootable flags.  This patch also refactors
_create_volume_from_snapshot() and _create_from_source_volume() to only
do Glance metadata copy when needed (bootable is True).

Fix bug: 1211632

Change-Id: I28f065e21cf24c81f98b00e171fcbe4f64ec76b6
Merge "Raise exception when Glance metadata not found."