[Improvement]: Change L2VPN create vlans step more idempotent

[tjeerddie]

this was brought up in a comment in the PR to support vlanranges, which brought multiple ims calls into the step: #59 (comment)

this should also be done for the vlan removal in terminate workflow.

TODO's

• update L2VPN ims_create_vlans.
• update L2VPN ims_remove_vlans.