GlobalVisualStudio10Generator: Support non-standard toolset json flag files.

Followup to converting to json files for allowing json flag sets other than the standard Visual Studio ones for custom toolsets.

If given a toolset that does not have an explicit mapping in cmVisualStudio10ToolsetOptions, check for a json flag file using the toolset name before trying the default toolset for the generator. Currently this is looking in the same location as the standard generated json files for Visual Studio for these custom toolset json files.