[bgpvpn] Make _create_loginable_secgroup_rule public

As tempest.scenario.manager was announced stable interface in Tempest
27.0.0[1] it can be now reused in plugins.

_create_loginable_secgroup method isn't ready to be deleted yet, because its rulesets differs from tempest's.
This method will be public now, so replaced methods will use this method over the tempest's one.
Tempest didn't allow to modify rulesets, but patch that allows this is already merged[2].
In some time, this plugin's implementation will be replaced

Etherpad concerning this effort:
https://etherpad.opendev.org/p/tempest-scenario-manager-cleanup

[1] https://docs.openstack.org/releasenotes/tempest/v27.0.0.html#release-notes-27-0-0
[2] https://review.opendev.org/c/openstack/tempest/+/835018

Change-Id: Ia2f8eed6da612bc88c9b9eb48873dcc11b309f42
1 file changed