Enable SSL for connections to database

Since the database can be exposed to the outer world, it seems better to
have an encrypted connection to it.
We automatically generate a self-signed certificate since it's only used
to encrypt the connection but one can easily replace it by a custom one,
just overwrite `server.crt` and `server.key` in the
/var/lib/dokku/services/postgres/DB_NAME/data directory.

tests/setup.sh

@@ -15,6 +15,7 @@ cd -
 rm -rf $DOKKU_ROOT/plugins/service
 mkdir -p $DOKKU_ROOT/plugins/service
 find ./ -maxdepth 1 -type f -exec cp '{}' $DOKKU_ROOT/plugins/service \;
+cp -r ./scripts "$DOKKU_ROOT/plugins/service"
 
 if [[ ! -f $BIN_STUBS/plugn ]]; then
   wget -O- "$PLUGN_URL" | tar xzf - -C "$BIN_STUBS"