Fixes purge jitsi-meet-prosody (missing debconf configs).

debian/jitsi-meet-prosody.config

+#!/bin/sh -e
+
+# Source debconf library.
+. /usr/share/debconf/confmodule

debian/jitsi-meet-prosody.postinst

 
         . /etc/jitsi/videobridge/config
 
+        # loading debconf
+        . /usr/share/debconf/confmodule
+
+        # stores the hostname so we will reuse it later, like in purge
+        db_set jitsi-meet-prosody/jvb-hostname $JVB_HOSTNAME
+
+        # and we're done with debconf
+        db_stop
+
         PROSODY_CONFIG_PRESENT="true"
         PROSODY_HOST_CONFIG="/etc/prosody/conf.avail/$JVB_HOSTNAME.cfg.lua"
         # if there is no prosody config extract our template

debian/jitsi-meet-prosody.postrm

     ;;
 
     purge)
-        db_get jitsi-meet/jvb-hostname
+        db_get jitsi-meet-prosody/jvb-hostname
         JVB_HOSTNAME=$RET
         if [ -n "$RET" ]; then
             rm -f /etc/prosody/conf.avail/$JVB_HOSTNAME.cfg.lua

debian/jitsi-meet-prosody.templates

+Template: jitsi-meet-prosody/jvb-hostname
+Type: string
+Default: ${default-key}
+_Description: The hostname of the current installation:
+ The value for the hostname that is set in Jitsi Videobridge installation.