Revert "Improved Ansible performance for post 2.0 releases."

This reverts commit 5fe5177f

For now the mitogen don't work on 2.5 which is the version installed by more recent centos.

/reviewed-on nexedi/slapos.package!52

playbook/ansible.cfg

 [defaults]
-strategy_plugins = /usr/share/ansible_plugins/mitogen/ansible_mitogen/plugins/strategy
-strategy = mitogen
+
 callback_plugins   = /usr/share/ansible_plugins/callback_plugins