move SECRET_KEY out of settings.py

[jdugan1024]

The Django SECRET_KEY should be set to a distinct secret value for each instance of esmond. Currently there is a single key in settings.py. Move this setting to esmond.conf and provide docs on generating it.

[arlake228]

Under utils of the andy-perfsonar-ma branch there is a script called gen_secret_key that might be useful. The rpm runs it but you can also run it by hand.