Using white-spaces while setting the SSL mode might cause query failures

Very minor issue, I don't think it'd be an actual problem in production systems. Mostly opened for not to lose track of it.

Postgres allows while spaces around [conn params](https://www.postgresql.org/docs/10/libpq-connect.html#LIBPQ-CONNSTRING), see `33.1.1.1. Keyword/Value Connection Strings`. However, if a user manually sets it for `citus.node_conninfo`, distributed queries fail.

```SQL
alter system set citus.node_conninfo TO "sslmode = require" ;
-- restart nodes
SELECT count(*) FROM test;
….
WARNING: connection error: localhost:9702
DETAIL: invalid connection option "postgres"
ERROR: failed to execute task 3
```