greeter::fixed the 'Login' text in the login button of guest donot in center

tower:https://tower.im/projects/8a6342c556be4c4eb898fdd7fd6fcb2f/todos/f36d971713fb49f1a3e26b12b1535ce1/

Change-Id: I37b73fa051c7c63198d717eeb461ff7b0df2d1e9