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

internal/nomad/api_querier_mock.go

@@ -42,7 +42,7 @@ func (_m *apiQuerierMock) AllocationStream(ctx context.Context) (<-chan *api.Eve
 }
 
 // DeleteRunner provides a mock function with given fields: runnerID
-func (_m *apiQuerierMock) DeleteRunner(runnerID string) error {
+func (_m *apiQuerierMock) DeleteJob(runnerID string) error {
 	ret := _m.Called(runnerID)
 
 	var r0 error