Update comments
Left align the legend to its column container

Note that "horizontalAlignment" is a new parameter in MapFish print
2.0-SNAPSHOT, and did not exist in 1.2.1-GEORCHESTRA version used
until georchestra/georchestra@2253e1b7.

Unfortunately, the parameter default has been set to "center", which
breaks compatibility with previous behavior. We then need to
explicitely set the parameter in confg.yaml.