jicofo: set XMPP_MUC_COMPONENT_PREFIX

Fixes: #691

jicofo/rootfs/defaults/sip-communicator.properties

@@ -18,6 +18,10 @@ org.jitsi.impl.reservation.rest.BASE_URL={{ .Env.JICOFO_RESERVATION_REST_BASE_UR
 org.jitsi.jicofo.health.ENABLE_HEALTH_CHECKS=true
 {{ end }}
 
+{{ if .Env.XMPP_MUC_DOMAIN }}
+org.jitsi.jicofo.XMPP_MUC_COMPONENT_PREFIX={{ first (splitList "." .Env.XMPP_MUC_DOMAIN) }}
+{{ end }}
+
 {{ $ENABLE_AUTH := .Env.ENABLE_AUTH | default "0" | toBool }}
 {{ $AUTH_TYPE := .Env.AUTH_TYPE | default "internal" }}