Using unicode() to handle image's properties

Replace str() to unicode() to allow nova-api work well with the fix I
did in glanceclient (https://review.openstack.org/#/c/29245/6).

Fixes: Bug #1180377

Change-Id: I31e532bb50e12e355b9dd26c9ce1e129da858cbe
Signed-off-by: Zhi Yan Liu <lzy.dev@gmail.com>
Merge "Using unicode() to handle image's properties"