Implement routes to list, get and delete execution environments

* #9 Implement routes to list, get and delete execution environments.
A refactoring was required to introduce the ExecutionEnvironment interface.

* Fix MR comments, linting issues and bug that lead to e2e test failure

* Add e2e tests

* Add unit tests

tests/e2e/e2e_test.go

@@ -65,7 +65,7 @@ func TestMain(m *testing.M) {
 	<-time.After(10 * time.Second)
 
 	code := m.Run()
-	cleanupJobsForEnvironment(&testing.T{}, "0")
+	cleanupJobsForEnvironment(&testing.T{}, tests.DefaultEnvironmentIDAsString)
 	os.Exit(code)
 }