workflows: add validate function to workspace_root_path

Goes on top of reanahub/reana-client#545 PR set, and reanahub/reana#532 PR set

Closes reanahub/reana-client#546

reana_server/rest/workflows.py

@@ -19,6 +19,7 @@
 from reana_commons.config import REANA_WORKFLOW_ENGINES
 from reana_commons.errors import REANAQuotaExceededError, REANAValidationError
 from reana_commons.operational_options import validate_operational_options
+from reana_commons.workspaces import validate_workspace
 from reana_db.database import Session
 from reana_db.models import (
     InteractiveSessionType,
@@ -401,8 +402,8 @@ def create_workflow(user):  # noqa
         workflow_dict["operational_options"] = validate_operational_options(
             workflow_engine, reana_spec_file.get("inputs", {}).get("options", {})
         )
-        workspace_root_path = reana_spec_file.get("workspace", {}).get(
-            "root_path", None
+        workspace_root_path = validate_workspace(
+            reana_spec_file.get("workspace", {}).get("root_path", None)
         )
         if git_data:
             workflow_dict["git_data"] = git_data