An empty string is an acceptable value of this entry, and it forces
logging to stdout/stderr, which is useful when running on containers.
In other modules (such as puppet-keystone) log_file defaults to
$::os_service_default. This is not the case in this module, so we
need to allow an empty value in log_file here as well.
Change-Id: I3fa4a38d21f0f7e447157ab7814a547c10a4b7d3