Tutorial examples are not generic

[vkarak]

I am running on a system where I have configured a cuda programming environment and I am getting the following error:

FAILURE INFO for Example2bTest
  * System partition: puhti:gpu
  * Environment: cuda
  * Stage directory: /users/training004/reframe/stage/puhti/gpu/cuda/Example2bTest
  * Job type: batch job (id=None)
  * Maintainers: ['you-can-type-your-email-here']
  * Failing phase: compile
  * Reason: unexpected error: 'cuda'
Traceback (most recent call last):
  File "/users/training004/reframe/reframe/frontend/executors/__init__.py", line 146, in _safe_call
    return fn(*args, **kwargs)
  File "/users/training004/reframe/reframe/core/pipeline.py", line 60, in _fn
    h(obj)
  File "/users/training004/reframe/reframe/core/decorators.py", line 180, in _fn
    func(*args, **kwargs)
  File "/users/training004/reframe/tutorial/example2.py", line 62, in setflags
    self.build_system.cflags = self.prgenv_flags[self.current_environ.name]
KeyError: 'cuda'

We should make the tutorial checks generic and also update the documentation.