Mongo shell `--host` option for replica sets broken in 3.4

[This commit](https://github.com/mongodb/mongo/commit/88b540f0c14c7ab8af708aecc4cd6df83081b32e) seems to have broken using replica sets with the `--host` option, which is documented [here](https://docs.mongodb.com/manual/reference/program/mongo/#cmdoption--host). I currently get the following error when trying to use this option: "Error: Failed to parse mongodb:// URL: mongodb://default_set_3.4/172.17.0.7:27017,172.17.0.5:27017,172.17.0.6:27017". Note that this works on MongoDB versions as far back as 2.4, and yet is broken on 3.4. The root cause of this feature breaking in 3.4 seems to be the lack of testing of this option.