externalbackend: refactor

[dprelipcean]

Similar to #125, refactor the reana ExternalBackend to inherit from the yadage ExternalAsyncMixin

The configuration part of the submit method would then be part of the inherited make_external_job method:

        parameters, state = finalize_inputs(parameters, state)
        job = build_job(spec['process'], parameters, state, self.config)

        prettified_cmd, wrapped_cmd = get_commands(job)

        image = spec['environment']['image']
        # tag = spec['environment']['imagetag']

        kerberos = False
        compute_backend = None
        resources = spec['environment'].get('resources', None)
        if resources:
            for item in resources:
                if 'kerberos' in item.keys():
                    kerberos = item['kerberos']
                if 'compute_backend' in item.keys():
                    compute_backend = item['compute_backend']

        log.info('state context is %s', state)
        log.info('would run job %s', job)

        state.ensure()

        log.info('submitting!')

        workflow_uuid = os.getenv('workflow_uuid', 'default')
        job_request_body = {
            'workflow_uuid': workflow_uuid,
            'experiment': os.getenv('REANA_WORKFLOW_ENGINE_YADAGE_EXPERIMENT',
                                    'default'),
            'image': image,
            'cmd': wrapped_cmd,
            'prettified_cmd': prettified_cmd,
            'workflow_workspace': os.getenv('workflow_workspace', 'default'),
            'job_name': metadata['name'],
            'cvmfs_mounts': MOUNT_CVMFS,
        }

        if compute_backend:
            job_request_body['compute_backend'] = compute_backend
        if kerberos:
            job_request_body['kerberos'] = kerberos