Updated "local" DB option to if case

[Devika-NH]

Adding one more db option-"local" for authentication

[viveksd87]

Hi Devika,
Thank you for submitting a pull request.
But I have a question, is the change made for a specific case/scenario?
Because, based on an issue raised earlier, issue#5, we were planning to get rid of the hard coded database name and make it configurable from the command line using option --authDb.
So, the change made as per the PR might not be in sync with the solution we want to give.

Please let me know if you have any other thoughts on it.

Thanks,
Vivek