fix:use host if hostReplicaSet is not provided

[NthMetal]

Change was for MongoDriver's buildConnectionUrl method for building a connection url for a replica set.
Currently the host that is used is the config option called "hostReplicaSet"
The change updates this so that if this option is not provided it uses a provided "host" with the given port.
It also defaults to using "127.0.0.1" as a host if none is provided, and "27017" if no port was provided.

This change was needed because I did not provide a connection url to my typeorm config, but I provided all other neccessary info to build a connection url. However, I provided only a "host" and not "hostReplicaSet" which caused it to create a url with the host "undefined". I tested this on an environment with a replica set.

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run lint passes with this change
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• Documentation has been updated to reflect this change
• The new commits follow conventions explained in CONTRIBUTING.md

[AlexMesser]

thank you for contribution!