added clickhouse as supported db (#5522)
added clickhouse as supported db (#5522)

I believe clickhouse is supported now since #1844 was merged
zhouyao1994/incubator-superset,airbnb/superset,mistercrunch/panoramix,airbnb/caravel,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,mistercrunch/panoramix,zhouyao1994/incubator-superset,airbnb/caravel,apache/incubator-superset,apache/incubator-superset,mistercrunch/panoramix,airbnb/superset,airbnb/superset,airbnb/caravel,airbnb/caravel,apache/incubator-superset,airbnb/superset,apache/incubator-superset,mistercrunch/panoramix,apache/incubator-superset