This patch introduces support for requests where the user does
not know the segmentation details of a subport and by specifying
segmentation_type=inherit will let the trunk plugin infer these
details from the network to which the subport is connected to, thus
ignoring the segmentation_id in case it were to be specified.
This type of request is currently expected to have correct results
when the network segmentation type is 'vlan', and the network has
only one segment (provider-net extension use case).
DocImpact: Extend trunk documentation to include Ironic use case.
Closes-bug: #1648129
Depends-on: Ib510aade1716e6ca92940b85245eda7d0c84a070
Change-Id: I3be2638fddf3a9723dd852a3f9ea9f64eb1d0dd6
4a6d06550b