Migrate to logins using Salsa

• Added OmniAuth::Builder with configurations of developer and gitlab strategies in Debci::SelfService as adding it here enables to use /user/auth/gitlab as route for Salsa Authentication.
• Added debci_salsa_client_id and debci_salsa_client_secret entry to lib/environment.sh and Debci::Config to specify these values for OmniAuth::Builder.
• Added routes /auth/gitlab/callback, /auth/failure to handle callback from Salsa.
• Added route /auth/developer/callback to handle login callback in developer environment.
• Added login.erb with both options of Login using Salsa and Login in Developer Environment (only present in developer environment) for /user/login route and updated /user/login accordingly.
• Added ruby-omniauth-gitlab in debian/control.
• Updated login function in self_service_spec.rb to login with the use of mock authentication hash.
• Added migration file to add column salsa_id to users table which is used to create new user record now and usernames are updated accordingly.
• removed definition of read_request_user in app.rb since its no longer required.
• Update in spec/spec_helper.rb to turn on test mode for OmniAuth.
• Updated tests in spec/debci/api.rb and spec/debci/self_service_spec.rb for new changes.