The create_master_db script needs to delete all blank users initially

Deleting `user=""` where `host="localhost"` doesn't seem to be enough, because deploying a new app with a different hostname will not have a host that is *literally* `localhost`

Read the final comment here:

https://github.com/rubber/rubber/issues/185#issuecomment-8578399
Merge pull request #369 from dannguyen/patch-1

The create_master_db script needs to delete all blank users initially