Conform with the upstream create_and_delete_subnets

Conform with the upstream create_and_delete_subnets

Ovn fails vs rally_full [1] but passes vs rally_jobs.
It reverts the small diff between rally_full and rally_jobs added by
"Update network context in selected rally tests". Then it may raise
side effects in case of L2-only testing [2]

[1] http://artifacts.opnfv.org/functest/JDD0PPKUOYO5/functest-ovn-opnfv-functest-benchmarking-cntt-jerma-rally_full_cntt-run-9/rally_full_cntt/rally_full_cntt.html
[2] https://github.com/opnfv/functest/commit/b4b2e7f9a5a3cb3379b45ac4ddd0ef949974a698

Change-Id: I5c7591a46de1b534b6b3dc511ca2caf20ac48e1f
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
(cherry picked from commit e1fee2d236dd923e8c5969f36bdc41922f1a8c56)