Untangle exclusive and enable_placement, update examples.

[mr0re1]

Motivations:

• Enable usage of placements with dense reservations.

• Encourage usage of placements by making them to always work regardless of nodeset configuration.

Done:
Remove constraints:

• "If a reservation is specified, var.enable_placement must be false" - dense reservations support external placements, non-dense reservation will ignore Slurm-created placement.

• "If any non-static nodesets has enable_placement, var.exclusive must be set true" - nodeset-wise (non-exclusive) placement can be used with dynamic nodes.

Remove enable_placement: false from SlurmV6 examples.

Reasons for users to set enable_placement=false:

• If non-dense reservation is used, user can avoid extra-cost of creating placement policies;
• If customer wants to avoid "all or nothing" VM provisioning behaviour;
• If customer wants to intentionally have "spread" VMs (e.g. for reliability reasons)

[alyssa-sm]

There is a error in PR-validation with one of the validate_golden_copy failing that should be addressed but otherwise LGTM