add used_cpus_per_task parameter

[smoors]

useful for the OSU test where the actually used cpus per task is different from the requested cpus per task

fixes #321

[smoors]

hm, this doesn't work for srun, where reframe automatically sets this:

srun --cpus-per-task=x ...

[smoors]

ok, i managed to fix `srun` by setting SLURM_CPUS_PER_TASK and using a custom launcher based on srun, or by monkeypatching it in the site config like this:

srunlauncher_init = SrunLauncher.__init__


def srunlauncher_patched_init(self):
    srunlauncher_init(self)
    self.use_cpus_per_task = False


SrunLauncher.__init__ = srunlauncher_patched_init

we could add this to the common config so it will work for everyone out of the box.

[smoors]

zut, setting SLURM_CPUS_PER_TASK doesn't work either:

srun: fatal: cpus-per-task set by two different environment variables SLURM_CPUS_PER_TASK=1 != SLURM_TRES_PER_TASK=cpu=2

i don't think it's wise to set/override SLURM_TRES_PER_TASK because it could also contain tres/gpu info.

[smoors]

ok, this time it's really fixed, by adding a custom srun launcher that sets the actually used cpus per task.