Proposal: Rename interface.type attribute to interface._type

[jbemmel]

Working on #1646 the deceptively simple request to "not define interface.type" poses some interesting challenges

1. I defined interface.type including "lag" to keep validation from failing
2. If we do not define interface.type, we need to ensure the lag module creates its interfaces after validation.
   This means refactoring (splitting) and doing the interface creation in module_pre_link_transform
3. Following that, the vlan module needs to be changed, because it expects link.interfaces to be populated by the time pre_link_transform runs. OK, so I refactored that to postpone that logic until link_pre_link_transform (merging in the existing method)
4. Now the vlan-trunk-attribute-error test case fails, because the validation logic no longer sees the user provided interface attributes (as processing has been postponed)

As you can imagine, all of the above is fixable, but it leads to a massive PR with lots of changes. And while I could split that up and separate out the vlan module changes, I'm thinking the easier and cleaner solution is to just rename "type" to "_type" and leave the rest as-is - until a final post-processing step where we rename _type to type such that the templates can remain the same

Thoughts? See #1653 for code impact

[jbemmel]

Or, alternatively, we could modify the validation routines and add some exception logic: If the attribute is in some YAML list, ignore it

That’s even simpler