Migrate instance_template modules from slurm-gcp repo

[mr0re1]

Move modules slurm_instance_template and _instance_template
from slurm-gcp/tree/master/terraform/slurm_cluster/modules
to community/modules/internal/slurm-gcp-v6.

Based on master branch at GoogleCloudPlatform/slurm-gcp@740472d

Non-breaking change, no need for moved-block, terraform and resource addressing is preserved completely.

Followup actions:

• Merge modules instance_template and internal_instance_template into one, currently blocked by google_compute_instance_template bug (to be reported);

• Lint and update license of community/modules/internal/slurm-gcp-v6/instance_template/files/startup_sh_unlinted (preserved unchanged to avoid drift).