[server][ConfigManager] Move error handling to guard clause

Because error case is unexpected case for the method. Guard if style
should use for unexpected cases. Main process (loading config file
successfully case for this method) should be at the normal indent level.
[server] Support custom MySQL authentication by config file

Here is a sample config file to custom MySQL authentication information:

    [mysql]
    database=hatohol-db1
    user=me
    password=secret
Merge pull request #530 from project-hatohol/server-support-db-config-in-config-file

[server] Support db config in config file