feat: enable bulk getting and unsetting of configuration options

[NucciTheBoss]

This pull request adds the ability to both bulk get and unset configuration options on slurm config objects. This functionality will be helpful for both if we want to reset the state of Slurm, and quickly get configuration information for various Slurm objects for comparison against new information received by the charms.

For example:

slurmctld = SlurmManagerBase(ServiceType.SLURMCTLD)

# Get the entire Slurm controller configuration
slurmctld.config.get()

# Reset the configuration
slurmctld.config.unset()

Misc.

• Fixes a bug with .unset(...). If we use snap unset ... instead of snap set ..., we don't need to append the ! exclamation point to the end of the key name. Whoopsies 🥶
• ConfigurationManager now takes a string in its constructor rather than a ServiceType. This makes it much easier to create configuration managers for slurmctld components such as cgroup and acct_gather. I realized that ConfigurationManager only needs config_name, so there's no need to actually pass the full ServiceType enum.

[NucciTheBoss]

I'll bump the LIBPATCH version after we discuss & approve

[jedel1043]

Agreed with all the changes, it makes it really easy to get/unset all the slurm config at once.
However, we were planning on adding some default config to fix some slurm limitations for the snap (charmed-hpc/slurm-snap#22), and I worry that using unset to clear everything could become a pitfall.

[NucciTheBoss]

Hmm... you make a point. Currently, slurmctld-operator defines the base set of configuration options it needs, so at least in the charm context we can just go back to that base state:

slurmctld = SlurmManagerBase(ServiceType.SLURMCTLD)
slurmctld.config.unset()
slurmctld.set_defaults()

From the snap context, if the entire configuration is blown out we could just add some logic to the configure hook that's like "if snap get -d slurm <option> is empty, revert to these defaults". How does that sound?

[jedel1043]

From the snap context, if the entire configuration is blown out we could just add some logic to the configure hook that's like "if snap get -d slurm <option> is empty, revert to these defaults". How does that sound?

Yep, sounds good!