[SARC-475] Handle datetime properly in slurm config parsing

[notoraptor]

Make sure GPUBilling and NodeGPUMapping objects use UTC datetimes (and not str datetimes anymore).

NB: We still cache slurm config files using local day, since we want to remember when (in local time) did we download these files.

[abergeron]

This looks ok, but I think that we should use UTC even in user-facing date parsing. Otherwise it might get confusing.

Also, this should absolutely get split into fetch and parse to avoid the awkwardness of "use cache if day is specified otherwise go fetch". It would also enable us to use the complete datetime for the cache timestamp because users wouldn't have to guess at the exact second the file was fetched to use the cache.

I'm ok with these things getting pushed to a second PR if you want to merge this first.

[notoraptor]

@abergeron OK ! I made a ticket about using complete datetime in UTC format to cache slurm files: https://mila-iqia.atlassian.net/browse/SARC-477

I will merge this PR and make a second PR later (this week or the next one).