Modify redmine auth

Force use API key for authentication of Redmine.

The current code supports both user/password style authentication & API key style one.
To use API key, you can use it as user name and enter random password. 
(You can find your API key from "My account" page.)

But I don't want manage raw password by Hatohol.
So this commit hides user/password style authentication & shows only API key style one.
(REST API still accepts both style.)

Because the DB table isn't Redmine specific, I don't remove "password" column from it.