Use ComputeTarget instead of Environment because of ambiguity

[EmielStoelinga]

Multiple files mention using or creating an "Environment" while in fact they refer to a ComputeTarget. Because of ambiguity, I would suggest to change the names of those variables and files. Afterwards, the template could be extended with the possibility to select a specific environment from the workspace for an experiment.

Examples:

• tasks/make_environment.py: actually creates a new compute target in the workspace instead of a new environment.
• tasks/train_model.py: environment argument actually refers to a compute target in the workspace.