Unify the code path of creating segment

[Hong Hui Xiao]

Before routed network, functions in [1] are used to manage network segment. Routed network introduces a plugin [2] to manage network segment. So, now there are 2 code paths to do the same job.

This cause issue when create network in ml2. When the network is created, related segmentation_id will be reserved in ml2 and related segments will be created by using [1]. In [1], PRECOMMIT_CREATE event for SEGMENT will be sent out.

In patch [3], a subscriber was added in ml2 to subscribe the PRECOMMIT_CREATE event of SEGMENT. The subscriber will reserve the segmentation_id.

But the segmentation_id has already been reserved. A workaround was added at [4] to avoid issue.

Ideally, ml2 should use [2] to create segment and let [2] to do other things(like reserve segmentation_id). This can eliminate the workaround.

[1] neutron.db.segments_db
[2] neutron.services.segments.plugin
[3] f564dcad4d8c072767ae235353a982653b156c76
[4] https://github.com/openstack/neutron/blob/8dffc238759cf543681443a2a4540dd0d569da6a/neutron/plugins/ml2/plugin.py#L1814-L1821

[Hong Hui Xiao (xiaohhui)]

I add some code at [1] and will use [1] to track the progress.

[1] https://review.openstack.org/#/c/359147/