Fix unicode key of azcache can't be stored to memcache

The memcache library we are using assumes all strings passed to
it are plain ascii, so we need to convert unicode to string.

Fix bug #1212164

Change-Id: Ibf36e6b096893cb9f056371bb43cbb441cae6e32
Merge "Fix unicode key of azcache can't be stored to memcache"