Add support for shared reservations

[mr0re1]

• Allow to specify reservation_name in "long format": projects/<project>/reservations/<reservation_name>
• Add constraint If a reservation is specified, `var.zones` should be empty.

Depends on GoogleCloudPlatform/slurm-gcp#152 - Have to be landed together

Testing done:

...
  - id: static_nodeset
    source: community/modules/compute/schedmd-slurm-gcp-v6-nodeset
    use: [network]
    settings:
      node_count_static: 2
      node_count_dynamic_max: 0
      enable_placement: false  # placement is done on reservation
      reservation_name: X
      machine_type: n2-standard-2
...

• Specific reservations without placement policy

  • Shared by full name - OK
  • Shared by short name - deploy error - OK
  • Local by full name - OK
  • Local by short name - OK

• Specific reservations with placement policy

  • Local by full name - OK
  • Local by short name
  • Shared by full name - not supported - OK
  • Shared by short name - not supported - OK
  

• Automatic reservations

  • Local by short name - deploy error - OK
  • Local by full name - deploy error - OK
  • Shared by short name - deploy error - OK
  • Shared by full name - deploy error - OK