If you have a relation and configuration option that are complimentary, this is one way to track those.

config-changed

#!/bin/bash

# Just a common function file to rebuild the configuration
. lib/functions

master_server=`config-get master-server`

# Have the install script create a servers.d directory
# Store all the different values in there
if [ -z $master_server ]; then
  rm -f servers.d/config
else
  echo "$master_server" > servers.d/config
fi

# This lives in lib/functions - or wherever you want.
update_configuration

functions

#!/bin/bash

update_configuration() {
  servers=""
  for server in `ls servers.d`; do
    $servers="$servers;$server"
  done

  # Us ch_template_file, sed, or some other means to put that
  # variable in to your configuration file.
}

master-relation-broken

#!/bin/bash

# Just a common function file to rebuild the configuration
. lib/functions

rm -f servers.d/$JUJU_RELATION_ID

# This lives in lib/functions - or wherever you want.
update_configuration

master-relation-changed

#!/bin/bash

# Just a common function file to rebuild the configuration
. lib/functions

master_server=`relation-get private-address`

if [ -z "$master_server" ]; then
  juju-log "Waiting for relation details"
  exit 0
fi

# $JUJU_RELATION_ID is unique for each relation. This is 
# important for tracking later when you _remove_ a relation.
echo "$master_server" > servers.d/$JUJU_RELATION_ID

# This lives in lib/functions - or wherever you want.
update_configuration