Motivation: - allow to provision login group independently of "controller monolith"

Changes:

• Move login nodes infra provisioning into community/modules/internal/slurm-gcp/login module;
• Remove login specific fields from stored config.yaml;
• Don't provision login artifacts (startup scripts) in slurm_files modules;
• Add separate "${bucket}/login_group_configs/${name}.yaml" config file for login nodes;
• Add metadata key slurm_login_group for VMs to self identify;
• Block login node setup on availability of both general config and group-specific config.

Breaking change - requires recreation of login nodes template and instances

NOTE: This PR could have been made into non-breaking change by:

• Don't create separate internal/nodeset module, but for_each resource creation within controller module - doesn't break down monolith and makes it harder to reuse in other products;
• Don't add slurm_login_group metadata, but rely on VM name parsing - less clean, require moderate amount of refactoring of python-code.