[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.

[hanstrompert]

It is enough to add comments in the source that mention that the given code isn't idempotent and, when you want to use this code in production, should be refactored to make it idempotent. We do not want to spend the time now in learning the specifics of Netbox.

[hanstrompert]

Please add a section on idempotency to the readme and point to the code.