L2VPN failure handling process upon inter-domain removal

[italovalcy]

This is related to Issue #478 and PR #523, and also atlanticwave-sdx/sdx-end-to-end-tests#128

We discussed about what should be done when a NNI attribute is removed and this NNI attribute is related to an inter-domain link. During the weekly SDX meeting we discussed this again and we agreed with the following strategy:

1. When one side of the inter-domain link have the NNI attribute removed, the link will be marked as error and all the L2VPN will not be changed

2. when both sides of the inter-domain link have the NNI attribute removed, the link will be removed and the L2VPN will be moved away from that link (similar to the link down failure handler)

[italovalcy]

Added test_021/092 to validate this case: https://github.com/atlanticwave-sdx/sdx-end-to-end-tests/blob/6341181aafdd8f26ad21ba167966707c9198dece/tests/test_21_use_case_topology.py#L379-L497

[YufengXin]

migrated from #478