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.

config/code_ocean.yml.ci

@@ -10,5 +10,6 @@ test:
   prometheus_exporter:
     enabled: false
   runner_management:
+    strategy: poseidon
     url: https://runners.example.org
     unused_runner_expiration_time: 180

config/code_ocean.yml.example

@@ -12,6 +12,7 @@ default: &default
   prometheus_exporter:
     enabled: false
   runner_management:
+    strategy: poseidon
     url: https://runners.example.org
     unused_runner_expiration_time: 180