[feat] Enable modifications to ReFrame's environment per system basis

[vkarak]

This PR introduces the modules and variables parameters at the system level of the site configuration. This is useful in cases were a module is always needed to be loaded for launching jobs for example:

        'systems': {
            'foosys': {
                'modules': ['slurm']
                'hostnames': ['localhost'],
                'partitions': {
                    'compute': {
                        'scheduler': 'nativeslurm',
                        'environs': ['builtin-gcc'],
                    },
                }
            },

Notice that this modifies the ReFrame environment, whereas the same parameters defined for a partition do not.

Fixes #806.

[vkarak]

@bcfriesen This fixes the issue you had when trying to load a module providing Slurm. You may now specify this module as parameter at the system level (check the description of this PR). Can you tell me if this works for you? Passing -m option should not be necessary any more.

[victorusu]

lgtm, but I have a couple of questions.

[victorusu]

Are we raising empty?

[vkarak]

No, this means raise the current exception. But I should remove e, which I don't use.

[bcfriesen]

Hi @vkarak,

@bcfriesen This fixes the issue you had when trying to load a module providing Slurm. You may now specify this module as parameter at the system level (check the description of this PR). Can you tell me if this works for you? Passing -m option should not be necessary any more.

Great! This is one of the last outstanding bugs that we have to work around at NERSC. We're returning from a system-wide maintenance today, so I will test this as soon as the systems come back (today or tomorrow).

[bcfriesen]

Hi @vkarak, sorry for the delay. This patch indeed works for me, thanks!