cobbler: Don't set ansible_python_interpreter manually anymore

cobbler: Don't set ansible_python_interpreter manually anymore

Both cobbler hosts (cobbler.front.sepia.ceph.com and magna001.ceph.redhat.com) are now running versions of ansible that handle python interpreter discovery automatically.

Setting this manually was actually causing issues because some ceph-cm-ansible tasks get delegated to localhost (Like 'Clone the keys repo' in the Users role).  So when setting -e ansible_python_interpreter=/usr/bin/python3, ansible looks for that binary on the localhost.  magna001 doesn't have python3 so the playbook run would fail.

Signed-off-by: David Galloway <dgallowa@redhat.com>