Consistent naming scheme; memcached_host -> memcached_hostname

environment.sh uses hostname for everything, and both environment and
credentials provide a default of 'memcached', so this should also be one less
variable to care about.

installer/roles/local_docker/templates/credentials.py.j2

@@ -26,7 +26,7 @@ CHANNEL_LAYERS = {
 CACHES = {
     'default': {
         'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
-        'LOCATION': '{}:{}'.format("{{ memcached_host }}", "{{ memcached_port }}")
+        'LOCATION': '{}:{}'.format("{{ memcached_hostname }}", "{{ memcached_port }}")
     },
     'ephemeral': {
         'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',