Allow more environments

Currently if you set CI_ENV to anything other than "development", "testing" or "production" you get a 503 error.
Our project has two testing environments. One at our development office, one on the production server, running under a different subdomain.

Local development, dev-testing (in docker), production-server testing and real "production" all have differences in either the name or IP address of the database. Currently to work around this we stray from the official versions, because of custom workarounds to this problem.

There should really be no issue as to set the environment to any string we want and that environment's configuration files to be loaded from the usual "application/config/myenv".