[proxy] memcache backend - keys must be bytes, not text

[Allele Dev]

From: tests.unit.proxy.test_catalog.CatalogTest.test_simple (using memcached oslo.cache backend)

```
MemcachedStringEncodingError: Keys must be str()'s, not unicode. Convert your unicode strings using mystring.encode(charset)!
```

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/49388

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/49388
Committed: http://github.com/openstack/marconi/commit/92fad17cc72007abdda46a3b6a98ec26e486f9cc
Submitter: Jenkins
Branch: master

commit 92fad17cc72007abdda46a3b6a98ec26e486f9cc
Author: Alejandro Cabrera <email address hidden>
Date: Wed Oct 2 11:25:22 2013 -0400

    fix: encode keys before caching

    This patch addresses an error in the caching layer that prevented it
    from playing nice with backends like memcached that expect keys to be
    bytestrings rather than unicode strings. The change is made in
    oslo.cache to make its use as transparent as possible to users.

    Change-Id: Iee48e16393321ff7908807a9f0f7fa20480ec5f4
    Closes-Bug: 1230556