[Refactor]: Fix NSISTP and VLAN validation

[hanstrompert]

Please fix the following in the NSISTP product/workflows:

• do not administrate VLAN group in Netbox, VLAN administration is only done in the orchestrator database
• move vlan validation code from workflows/nsistp/shared/vlan.py to workflows/shared
• add vlan validation to create workflow of L2VPN product