Refactor e-mail handling resources

[OtisRed]

Subject of the issue:

After implementing #426 and #427 it would be good to clean up the files related to that function.

Technical description:

• In backend/config.py some of the code between 33 and 68 line will become redundant or possible to make more clear.
• In travis.yml it would be nice not to ask etheral email for credentials on production env (we don't test on production!)
• Some of these changes along with Create the method checking for the env variable #426 and Create a method handling boto3 API #427 have implications for Terraform in infrastructure repo.