check-compose: send Postrelease emails to appropriate folks

This is for https://phab.qadevel.cloud.fedoraproject.org/T690 ,
it goes along with some changes to check-compose itself to
allow reports for different milestones to have different config
settings, which (if I didn't mess anything up) should result in
those reports going to Mike McGrath and the cloud@ list, as
requested, while Rawhide and Branched reports continue to go to
test@ and devel@.

roles/check-compose/templates/check-compose.conf.j2

@@ -23,3 +23,7 @@ json: {{ checkcompose_json }}
 {% if checkcompose_loglevel is defined %}
 loglevel: {{ checkcompose_loglevel }}
 {% endif %}
+[Postrelease]
+{% if checkcompose_postrelease_emailto is defined %}
+email-to: {{ checkcompose_postrelease_emailto }}
+{% endif %}