Adding the necessary variables to configure OAuth2 Client

These changes fix the next two issues:

• The login isn't working because some variables wasn't defined, so always the login redirect to http://127.0.0.1:8000, these variables were hardcoded in the file analytics_dashboard/settings/base.py

• In the file /edx/etc/insights.yml the variabels are defined with the prefix "INSIGHTS_" and that isn't working because We are expecting the variables without the prefix.

*reviewers:*
@morenol