Use ansible 2.12 for testcases_prepare (#8763)

tests/requirements.txt links to tests/requirements-2.12.txt, so
Kubespray uses ansible 2.12 by default for testing. However we
forgot to update testcases_prepare.sh to use ansible 2.12.
This updates testcases_prepare to use ansible 2.12.

tests/scripts/testcases_prepare.sh

 #!/bin/bash
 set -euxo pipefail
 
-: ${ANSIBLE_MAJOR_VERSION:=2.10}
+: ${ANSIBLE_MAJOR_VERSION:=2.12}
 
 /usr/bin/python -m pip uninstall -y ansible ansible-base ansible-core
 /usr/bin/python -m pip install -r tests/requirements-${ANSIBLE_MAJOR_VERSION}.txt