The default value of SECRET_KEY was hard-coded in the settings.py which would turn out to be used all the time even if people are not forced to change it. So this commit removes that value and instead gets the SECRET_KEY from the environment variable.
Closes#99
Provided MAILMAN_DEFAULT_DOMAIN environment variable, rename example.com with
the provided domain name. Fix a bug where if the default username exists carry
on without creating the superuser.
Reuse the SERVE_FROM_DOMAIN instead of new MAILMAN_FROM_DOMAIN variable to set
the default Django SITE.
