"glance image-tag-update" command returns unicoded response
for lists. Therefore it requires print_list method from util
class to handle such case. Added unicode_key_value_to_string()
method to remove extra 'u' from lists and dictionaries. This
fix is inspired from cinderclient's implementation.
Change-Id: I16a04e8d34f7629f72fe389456001ca1db9335ea
Closes-bug: #1534046