Accept empty strings for log_dir
Accept empty strings for log_dir

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: I9d5b4e6c0fb0fa238e9a12494790c0fa5a83dd5b

CiscoSystems/puppet-neutron,CiscoSystems/puppet-neutron,fuel-infra/puppet-neutron,openstack/puppet-neutron,CiscoSystems/puppet-neutron,fuel-infra/puppet-neutron,openstack/puppet-neutron,fuel-infra/puppet-neutron