[DBAgentMySQL] Set MYSQL_READ_DEFAULT_GROUP to hatohol

So that Hatohol can read "client" group and "hatohol" group in
my.cnf.
Users should set default-character-set=utf8 (or utf8mb4) to
one of these groups to handle UTF-8 correctly.
[travis] Set the character encoding of MySQL
Merge pull request #849 from project-hatohol/mysql-charset

Load global my.cnf