Convert non-ascii characters within image property to unicode

Convert non-ascii characters within image property (key/value pair) to
unicode but utf-8 to prevent provisioning failure when cloud using qpid
backend.
This change also make the image property encoding consistency between
the image updating and the receiving. Before this, image property
updating use unicode, but receiving (get) result is utf-8.

Fixes: Bug #1180377

Change-Id: I010760c598a7e008c79f1240255708265352cdb5
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>