populate_db: Add option to specify max no of topics to create.

This will help us create more topics per stream than we currently do
by default.

tools/rebuild-test-database

@@ -33,6 +33,7 @@ create_zulip_test
 
 # This next line can be simplified to "-n0" once we fix our app (and tests) with 0 messages.
 ./manage.py populate_db --test-suite -n30 --threads=1 \
+    --max-topics=3 \
     --huddles=0 --personals=0 --percent-huddles=0 --percent-personals=0
 
 ./manage.py dumpdata \