[Enhancement] Way to setup config from stdin and/or file

[jedel1043]

As an improvement to integrate with the Slurm charms, we need to have a way to set multiple configurations using either input or a file. This should make it really easy to bulk-modify a lot of settings without having to call snap set slurm for each one of the modified setting.

[NucciTheBoss]

I'm going to close this one out. We discussed over Mattermost that snap set ... enables us to set multiple configs at once so that we don't have to call snap set slurm ... one-by-one for each configuration we want to update:

$ snap set slurm config.value1=... config.value2=... config.value3=... ...

I think it could potentially be an interesting discussion for us to add the ability to export your configuration from one snap to and import into another, but I think the charmed operators and snap get -d enable us to do this pretty easily right now for our needs, so I think this discussion should happen later down the line and potentially in a separate issue.