Set segments.segmentation_id as computed on networks

[nikParasyr]

Segmentation_id on openstack_networking_network_v2:

1. is statically defined by the user: in this case any changes on it should trigger a recreation of the network.
2. is not defined by the user but neutron has an available pool: in this case, neutron picks a segmentation_id from the pool and allocates it for this network. Thus the value is computed by neutron.

Because use-case 1 AND 2 are valid (depending on the cloud configuration) we cannot use something like DiffSuppressFunc. Setting it as computed should work in both cases.

Unfortunately i dont think we can easily test this in the ci.

Close #1560