fixup! stack/erp5 += WCFS service (technology preview)

In commit 48b24182 I passed 'wcfs_enable' to instance-zope.cfg as string
instead of an object. As the result, even if it was wcfs_enable=False, in
instance-zope.cfg in

    ---- 8< ---- https://lab.nexedi.com/nexedi/slapos/blob/8d3af101/stack/erp5/instance-zope.cfg.in#L68-72)
    {% if slapparameter_dict['wcfs_enable'] %}
      WENDELIN_CORE_VIRTMEM=r:wcfs+w:uvmm
    {% else %}
      WENDELIN_CORE_VIRTMEM=rw:uvmm
    {% endif %}

it was always taking the first branch, becuse slapparameter_dict['wcfs_enable']
was giving 'False' - a string - whose boolean value is True

    In [1]: bool('False')
    Out[1]: True

-> Fix it by passing slapparameter_dict['wcfs_enable'], similarly to most other
keys, as object.