Allow geowebcache to query geoserver for infoformat mime type list

[prushforth]

I had to hard-code the text/mapml mime type into 30cdbbe so that gwc won't refuse to do a featureinfo for text/mapml. This works great in the MapML preview, but having it hard-coded into gwc doesn't allow a geoserver not to have the MapML Community Module installed.

Either I can't support text/mapml as a GetFeatureInfo / FeatureInfo format in cached layers until MapML becomes a core module, OR gwc should be made to query geoserver (perhaps once at startup) to get the list of info formats supported by the back end WMS. Then, GWC could condition its capabilities document accordingly, instead of including text/mapml unconditionally.