Use variables to define workshop specific values

[Sabryr]

This is to use the _config.yml (as in hpc-in-a-day lesson) to define, for example queing system used
e.g from hpc-in-a day

this is the scheduler to be used for the workshop
possible values: lsf, slurm, pbs
workshop_scheduler: "slurm"
workshop_login_host: "cray-1"
workshop_shared_fast_filesystem: "/fastfs"

@aturner-epcc is working on a full solution for this. But due to time constraint I will fix the first few lessons (as it is easier than manually editing from my part) on my folk.

[psteinb]

As mentioned in #79, this is tackled in #73 . I propose to review this one and merge it. Feel free to suggest alternatives so we can start a discussion on them. I am a bit irritated by this issue and some comments on #79. Does this all mean, that people don't like #73? If so, could you please comment on it directly. Otherwise, I'd suggest to close this issue and sort out with @aturner-epcc and others who works on what. From what I read, I hope we are not working at the same thing at the same time.

[Sabryr]

Sorry my bad, I overlooked "Rewrite of cluster lesson #73" as it is so broad. Git grep show that you have the workshop variables in it as well-. Hope you could push this PR through soon

[psteinb]

Superb. Thanks for clarifying this.