Move logrus setup code to log package

[mxpv]

This PR reduces the spread of logrus imports. It moves logrus setup code to the log package to simplify migration to a new logger in the future (containerd/log#2)

[Iceber]

Is it better if the type of the parameter is Level? so that when using it, we can call log.SetLevel(log.InfoLevel)

Of course, we can also add a check for the level parameter, eg.

// SetLevel sets log level globally.
func SetLevel(level Level) error {
        lvl, err := logrus.ParseLevel(level.String())
        if err != nil {
                return err
        }

        logrus.SetLevel(lvl)
        return nil
}