This patch enables shared parameter similarly to patch [0],
only on test class level.
For podified environments some tests will temporary enable 'shared'
parameter for external network. This is because it was decided that
on podified environments external network should not be shared
by default for some tempest api tests to work properly, see [1].
Podified environment should run all the tests that modify external
network one by one, in a single tempest thread.
[0] eacd16a847
[1] https://github.com/openstack-k8s-operators/ci-framework/pull/1862
Change-Id: Id841b5c1cb5b620d8fcf8852b6041b1f80257505