Update cache prefix using EDXAPP_ENV_EXTRA_VARS.
Update to have a new flag for updating the staticfiles KEY_PREFIX.
Update cms and preview env configs.
Use the build number instead of the time for the cache key.
Add the triple to make the key more unique.
Update triple order.
Merge pull request #858 from edx/feanil/always_update_cache_prefixes

Feanil/always update cache prefixes