greeter::guest login button update

1.update guest login-button image
2.innerText is 'Login' and msgstr is '登录'
tower:https://tower.im/projects/8a6342c556be4c4eb898fdd7fd6fcb2f/todos/8c20d8bf3cfb4a85b71f0665c9bcbcbd/#b1bd0dc573fc43ec8fce6acd8035e1e4

Change-Id: I8d808db40d6af36e4fea31f0c08cd1fce81ded8c