Hardcoded random-password-generator-service URL breaks user setup API

[Link to code](https://github.com/ohmage/server/blob/master/src/org/ohmage/request/user/UserSetupRequest.java#L466)

Alternatives: 
1) Allow URL to be overridden with configuration option
2) Remove depedence on an external API altogether: generate random passwords within Ohmage