Fix typo on cache backend module

The correct module is 'dogpile.cache.memcached' and not
'dogpile.cache.memcache'.

It's important to backport this to Icehouse as all Juno
docs use the icehouse sample at this point. For example:
http://docs.openstack.org/trunk/config-reference/content/section_keystone.conf.html

Change-Id: Ie452c95dab4fcd04f65f6378c735490ee8cf4c70
(cherry picked from commit b8bbab790227445d90e4e9201127413bc85c1649)
Merge "Fix typo on cache backend module" into stable/icehouse