[sfc] Remove _create_subnet & _check_tenant_network_connectivity

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

Removed methods:
    * _create_subnet
    * _check_tenant_network_connectivity

Implementation of tempest's create_subnet differs a little from
this one. The changes mostly consist of bug fixes.
See related patches.[2][3][4]

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://opendev.org/openstack/tempest/commit/5b0a52fa5d626f67c1ace1e3a3993b9046767fba
[3] https://opendev.org/openstack/tempest/commit/4ad346e6497d4fbad9e379e8ed50730a0b6b55a3
[4] https://opendev.org/openstack/tempest/commit/795ef25b728964e37acb5e12761dfc0873fd2959

Change-Id: I672733833c9925b96244a2083d3972bf8424dca1
2 files changed