Standardize on newer except syntax

We currently have a mix of 'except X, y:' and 'except X as y:' syntax.
Standardize on the newer 'except as' form.

This patch was generated automatically using:

find * -type f -name 'glance-*' ! -name '*\.conf' ! -name '*\.ini' -o \
 -name '*\.py' | xargs 2to3 -f except | patch -p0

Addresses bug 1160331.

Change-Id: Ica8d1303973af64d0406457267ee75f608e869d0
Merge "Standardize on newer except syntax"