Role to configure mailman

**Role Variables**

.. zuul:rolevar:: mailmain_block_ua
   :default: unset

   Set to a list of strings of user agents to block via Apache config.
   Note this is a `RewriteCond directive
   <https://httpd.apache.org/docs/2.4/mod/mod_rewrite.html>`__, so
   for example to exactly match a string prefix it with ``=``.