Python toolbox should be able to advertise the Environment's they honor

[JacquesBeaurain]

Python toolbox tools should be able to advertise the environment's they honor in the same way as built-in tools do. Currently the Environments tab appear empty and any environment settings that the tool uses cannot be overridden unless the tool explicitly expose them as parameters (with defaults being the environment settings).

Compare built-in:

Versus custom Python tool:

 

Also see related  discussion in GIS Developers->Python