made docker.image_tag optional in msg to avoid image_tag="" fields
made docker.image_tag optional in msg to avoid image_tag="" fields

Change-Id: Ia4ee04aceea6a608e4e49cf38b431f90dd234dfd

nyxcharon/logspout-redis-logstash,schmunk42/logspout-redis-logstash,schmunk42/logspout-redis-logstash,rtoma/logspout-redis-logstash,nyxcharon/logspout-redis-logstash,chrisferry/logspout-redis-logstash,dkhunt27/logspout-redis-logstash,chrisferry/logspout-redis-logstash,dkhunt27/logspout-redis-logstash,brendangibat/logspout-redis-logstash,brendangibat/logspout-redis-logstash,rtoma/logspout-redis-logstash