Fix end-to-end tests for recent Synapse features (#8159)

* Ensure a useful logfile gets recorded for synapse startup failures

* Update synapse config to stop complaining about key servers

* Add flag to turn on registrations again

From https://github.com/matrix-org/synapse/pull/12091

test/end-to-end-tests/synapse/start.sh

@@ -5,11 +5,5 @@ BASE_DIR=$(cd $(dirname $0) && pwd)
 cd $BASE_DIR
 cd installations/consent/env/bin/
 source activate
-LOGFILE=$(mktemp)
 echo "Synapse log file at $LOGFILE"
 ./synctl start 2> $LOGFILE
-EXIT_CODE=$?
-if [ $EXIT_CODE -ne 0 ]; then
-	cat $LOGFILE
-fi
-exit $EXIT_CODE