mariadb: use /bin/false as the login shell

Use /bin/false as the login shell, just like what Ubuntu does,
otherwise there might be secure issue.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>