Restructure move to nested virt nodes

With [1] scenario jobs were moved to nested virt
nodes. But that also modified jobs for EM releases
like queens/rocky/stein/train/ussuri as all jobs have
common parent. And since these jobs override the
nodeset can fail with 'kvm' libvirt_type when
not using nested-virt nodes.

So proposing to create two variants of nested-switch
job based on Extended Maintenance or non Extended
maintenance releases. As only few providers provide
the nested virt nodes utilizing these nodes only
for non EM releases would be better as those
only have maximum number of patches.

Also adding zuul.d/base-nested-switch.yaml to irrelevant
files to allow easier switch to non nested virt nodes
when there is issue with all the providers providing
these nodes or when we see issues with nested-virt
nodes.

[1] https://review.opendev.org/c/openstack/neutron-tempest-plugin/+/821067

Change-Id: Ibee6b0bc7fc219f3d1e43795fa42b56512ee3bb8
3 files changed