[bgpvpn] Replace security groups related methods

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

Replaced methods:
    * _create_empty_security_group
    * _default_security_group (doesn't exist in tempest anymore)
    * _create_security_group_rule

create_loginable_secgroup_rule can't be replaced just yet[2].

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/neutron-tempest-plugin/+/837511

Change-Id: I6015457971858737e861f5fccc56dfe6ec2b9c77
2 files changed