Improve permission granularity scoping provider to pool

[ardubev16]

I've found myself wanting to manage just a part of my Proxmox cluster using Terraform. To avoid doing damages I created a Pool and gave the Terraform user the permissions specified in the doc just in that pool.

This however doesn't work since the provider only checks the permissions for the / path and my user doesn't have any (the permissions are instead in /pool/<my_pool>).

To handle this scenario I propose the following changes:

• a new provider variable pm_pool_scope to define the pool Terraform is supposed to use;
• all resources will also have the pool field populated by default (idk if it's feasible, I just started playing with Terraform) and will be created inside that pool;
• the permissions will be checked just for that pool and not globally.

In case this is a welcome proposal, I'd like to work on it! I took a quick look at the code and think there would be also the need to modify something in the proxmox-api-go library.