Modify link instantiation logic to handle various decommissioning states

[jonstout]

When oess decoms a node any connected links should also be decomd, but if the link still exists in nso then oess will try to recreate it. Because oess will always try to remain in sync with nso update our link creation logic such that:

• If a link with a given name doesn't exist create it
• If a link with a given name already exists and both its interfaces are on active nodes then the link should be updated and activated if currently decom'd.
• If a link with a given name already exists and either of its interfaces are on decomd nodes then no update should occur.