New horizon release use [1] for cache backend
instead of [2] as it was in previous versions.
This patch:
1. Removes override from config and
configure only memcached endpoints, not backend
specification itself. This will avoid bugs
in future in case BACKEND will be switched again.
2. Remove 'memcached' context from kolla_address filter
and use 'url' as [1] don't support inet6:[{address}]
for ipv6 but supports [{address}] which 'url' provides.
[1] django.core.cache.backends.memcached.PyMemcacheCache
[2] django.core.cache.backends.memcached.MemcachedCache
Change-Id: Ie3a8f47e7b776b6aa2bb9b1522fdd4514ea1484b
4108aea879