Move INSTANCE_NAME to local.py

Use it in more places as well
Move AUTH_LDAP_USER_DN_TEMPLATE to settings/__init__

The variable is meant to be constructed always from
AUTH_LDAP_USER_ATTR and AUTH_LDAP_USER_BASE_DN,
and never be touched
Merge pull request #8 from tampakrap/instance_name

Move INSTANCE_NAME to local.py