Fix potential missing key error in container_info

If upgrading from a non-storage policy enabled version of
swift to a storage policy enabled version its possible that
memcached will have an info structure that does not contain
the 'storage_policy" key resulting in an unhandled exception
during the lookup.  The fix is to simply make sure we never
return the dict without a storage_policy key defined; if it
doesn't exist its safe to make it '0' as this means you're
in the update scenario and there's xno other possibility.

Change-Id: If8c88f67ba7a3180ad06b586372fe35c65807aac