k8s: unify the label format

[audrium]

We should decide whether we want to use _ or - in k8s labels and whether we want to use the prefix reana.io/... (as we do for example for the reana.io/system label).

For reference:

reana-workflow-controller/reana_workflow_controller/workflow_run_manager.py

Lines 458 to 459 in 6bdd570

	"reana_workflow_mode": "batch",
	"reana-run-batch-workflow-uuid": str(self.workflow.id_),

Originally posted by @mdonadoni in #500 (comment)

[mdonadoni]

In addition to harmonising the format of the labels, new labels should be added to make it easy to filter run-batch/run-job/run-session pods by workflow ID/user ID (and maybe even user email?)