Patched nova/availability_zones.py

When Tim and I were working on rolling out BFG changes, we ran into an issue
where `nova-manage service list` would error with:

    2013-10-12 21:46:45,385.385 23317 TRACE nova MemcachedStringEncodingError: Keys must be str()'s, not unicode.  Convert your unicode strings using mystring.encode(charset)!

This has already been fixed upstream [1].  Rather than moving to havanah-rc2,
patching in place for now.

[1] https://review.openstack.org/#/c/41873/