Markusconfigurator

This is a pull request for simplifying MarkUs configuration. Now, specific configuration will be done in config/config.yml and not anymore in config/environments/<env>.rb. This is Rails best default way to do this.

I updated MarkusConfigurator in order to take into account config.yml.
Moreover, MarkusConfigurator doesn't set default values anymore. It was confusing in production. MarkusConfigurator will raise an error if a setting is missing.

This is the first part of the patch issued from Alan's (UofT) discussion. A second patch for routes with different instances on same subdomains will follow.

poke @danielstjules @karenreid 
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