client: `create_workflow_from_json` does not work with snakemake workflows

[mdonadoni]

When using the workflow_file parameter of create_workflow_from_json, snakemake workflows (and probably also the others) do not run correctly as the Snakemake specification is never loaded:

From reana-demo-helloword:

import os

import yaml
from reana_client.api.client import (
    create_workflow_from_json,
    upload_to_server,
    start_workflow,
)

WORKFLOW_NAME = "api-client-snakemake"
REANA_ACCESS_TOKEN = os.environ["REANA_ACCESS_TOKEN"]
REANA_YAML = "reana-snakemake.yaml"

with open(REANA_YAML) as f:
    spec = yaml.safe_load(f)

create_workflow_from_json(
    workflow_file=spec["workflow"]["file"],
    name=WORKFLOW_NAME,
    access_token=REANA_ACCESS_TOKEN,
    parameters=spec["inputs"],
    workflow_engine=spec["workflow"]["type"],
    outputs=spec["outputs"],
)

abs_path_to_input_files = [os.path.abspath(f) for f in spec["inputs"]["files"]]
upload_to_server(WORKFLOW_NAME, abs_path_to_input_files, REANA_ACCESS_TOKEN)

start_workflow(WORKFLOW_NAME, REANA_ACCESS_TOKEN, {})