Accept empty strings for log_dir and log_file
Accept empty strings for log_dir and log_file

An empty string is an acceptable value of this entry, and it forces
logging to stdout/stderr, which is useful when running on containers.

Change-Id: I363a7558e4addf6b984e27bfa19ebec6d13aa01d

openstack/puppet-trove,openstack/puppet-trove,openstack/puppet-trove