Introduce strategy for runner behavior

The runner model is only a class responsible for storing information
now. Based on the configuration it picks a strategy for the runner
management. The Poseidon strategy is already implemented and tested.
The Docker strategy will follow.

spec/factories/runner.rb

@@ -3,7 +3,7 @@
 # This factory does not request the runner management as the id is already provided.
 FactoryBot.define do
   factory :runner do
-    runner_id { 'test-runner-id' }
+    sequence(:runner_id) {|n| "test-runner-id-#{n}" }
     association :execution_environment, factory: :ruby
     association :user, factory: :external_user
     waiting_time { 1.0 }