stringify conditional before handing it to Jinja2 for evaluation.

This should fix simple conditionals like:

    when: ansible_selinux

While here add tests for ansible.utils.check_conditional() in
"jinja2_compare mode".

Without a fix similar to the proposed one, right now a playbook like the following fails on exception:

    - name: workingp
      hosts: all
      tasks:
        - name: works ?
          action: debug msg="yes or no"
          when: ansible_selinux