add compute limits as a data source

[ba-work]

This address my comment in #1289

Tested on a real OpenStack

openstack command:

$ o limits show --absolute --project 16c91b6234df4cebabbe704b4a6f2083
+--------------------------+-------+
| Name                     | Value |
+--------------------------+-------+
| maxServerMeta            |   128 |
| maxPersonality           |     5 |
| totalServerGroupsUsed    |     0 |
| maxImageMeta             |   128 |
| maxPersonalitySize       | 10240 |
| maxTotalKeypairs         |   100 |
| maxSecurityGroupRules    |    20 |
| maxServerGroups          |    10 |
| totalCoresUsed           |     8 |
| totalRAMUsed             | 16384 |
| totalInstancesUsed       |     3 |
| maxSecurityGroups        |    10 |
| totalFloatingIpsUsed     |     0 |
| maxTotalCores            |    20 |
| maxServerGroupMembers    |    10 |
| maxTotalFloatingIps      |    10 |
| totalSecurityGroupsUsed  |     1 |
| maxTotalInstances        |    10 |
| maxTotalRAMSize          | 51200 |
| totalSnapshotsUsed       |     0 |
| maxTotalBackups          |    50 |
| maxTotalVolumeGigabytes  |  1500 |
| maxTotalSnapshots        |    10 |
| maxTotalBackupGigabytes  |     0 |
| totalBackupGigabytesUsed |     0 |
| maxTotalVolumes          |   100 |
| totalVolumesUsed         |    35 |
| totalBackupsUsed         |     0 |
| totalGigabytesUsed       |  1238 |
+--------------------------+-------+

and tf:

brock@micro:~/os-test$ cat main.tf && tf plan
terraform {
  required_providers {
    openstack = {
      version = "0.1"
      source  = "registry.terraform.io/terraform-provider-openstack/openstack"
    }
  }
}
provider "openstack" { insecure = true }
data "openstack_compute_limits_v2" "source" { project_id = "16c91b6234df4cebabbe704b4a6f2083" }
output "info" { value = data.openstack_compute_limits_v2.source }

Changes to Outputs:
  + info = {
      + id                         = "16c91b6234df4cebabbe704b4a6f2083/regionOne"
      + max_image_meta             = 128
      + max_personality            = 5
      + max_personality_size       = 10240
      + max_security_group_rules   = 20
      + max_security_groups        = 10
      + max_server_group_members   = 10
      + max_server_groups          = 10
      + max_server_meta            = 128
      + max_total_cores            = 20
      + max_total_floating_ips     = 10
      + max_total_instances        = 10
      + max_total_keypairs         = 100
      + max_total_ram_size         = 51200
      + project_id                 = "16c91b6234df4cebabbe704b4a6f2083"
      + total_cores_used           = 8
      + total_floating_ips_used    = 0
      + total_instances_used       = 3
      + total_ram_used             = 16384
      + total_security_groups_used = 1
      + total_server_groups_used   = 0
    }

You can apply this plan to save these new output values to the Terraform state, without changing any real infrastructure.

──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

Note: You didn't use the -out option to save this plan, so Terraform can't guarantee to take exactly these actions if you run
"terraform apply" now.

I basically just copied the conventions in #1302, #1318, and #1319

This allows you to use data source custom conditions to make sure the target project has enough available quota for whatever you are about to deploy.

[nikParasyr]

It works. Thank you for picking it up :)