Avoid non standard WFS shapefile output

[fergonco]

Currently the installation is documented using this option[1], which makes the shapefile output non standard if it contains dates, as it is explained here[2].

Pay also attention to the parameter -Duser.timezone=GMT

[1] -Dorg.geotools.shapefile.datetime=true
[2] http://docs.geoserver.org/latest/en/user/tutorials/imagemosaic_timeseries/imagemosaic_timeseries.html#configuration