[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
diff --git a/neutron_tempest_plugin/bgpvpn/scenario/manager.py b/neutron_tempest_plugin/bgpvpn/scenario/manager.py
index 4a2df03..422df19 100644
--- a/neutron_tempest_plugin/bgpvpn/scenario/manager.py
+++ b/neutron_tempest_plugin/bgpvpn/scenario/manager.py
@@ -157,7 +157,7 @@
             tenant_id=tenant_id)
 
         # Add rules to the security group
-        rules = self._create_loginable_secgroup_rule(
+        rules = self.create_loginable_secgroup_rule(
             security_group_rules_client=security_group_rules_client,
             secgroup=secgroup,
             security_groups_client=security_groups_client)
@@ -257,9 +257,9 @@
 
         return sg_rule
 
-    def _create_loginable_secgroup_rule(self, security_group_rules_client=None,
-                                        secgroup=None,
-                                        security_groups_client=None):
+    def create_loginable_secgroup_rule(self, security_group_rules_client=None,
+                                       secgroup=None,
+                                       security_groups_client=None):
         """Create loginable security group rule
 
         This function will create: