Update cms_plugins.py

Added render_template shim to class definition - fixes ImproperlyConfigured Exception in Django-CMS 3.0RC1 "CMS Plugins must define a render template or set render_plugin=False: <class 'form_designer.contrib.cms_plugins.form_designer_form.cms_plugins.FormDesignerPlugin'>"
Update cms_plugins.py

Set to a sensible default, rather than a string.
Merge pull request #77 from estuans/master

Update cms_plugins.py