Validate that value is a valid heartbeat integer.

def is_heartbeat(value):
    """Validate that value is a valid heartbeat integer."""
    try:
        value = vol.Coerce(int)(value)
        return value
    except vol.Invalid:
        _LOGGER.warning(
            '%s is not a valid heartbeat value, falling back to heartbeat 0',
            value)
        return 0