load CWL and Snakemake specification

[VMois]

Originated from this analysis - #535 (comment)

With reana-client-go, we cannot load workflow specifications for CWL, Snakemake, and Yadage. Because of that, we will need to move specification loading to reana-server. Currently, Yadage is loaded in the start_workflow endpoint. We should also add loading of CWL and Snakemake specifications there. This change should nicely play with validation there.

Example of Yadage loading and other engines:

reana-server/reana_server/rest/workflows.py

Lines 562 to 572 in 4952c16

	# This is necessary for GitLab integration
	if workflow.type_ == "yadage":
	_load_and_save_yadage_spec(
	workflow, workflow_dict["operational_options"]
	)
	elif workflow.type_ in ["cwl", "snakemake"]:
	reana_yaml_path = os.path.join(workflow.workspace_path, "reana.yaml")
	workflow.reana_specification = load_reana_spec(
	reana_yaml_path, workflow.workspace_path
	)
	Session.commit()