Add new argument to Provider configuration to ignore metadata entries

[adambarreiro]

Closes #959
Requires https://github.com/vmware/go-vcloud-director/pull/581/files

This PR adds a new argument to the Terraform VCD Provider configuration: ignore_metadata_changes, which accepts the following sub-arguments:

• resource_type: Optional, specifies the resource type which metadata needs to be ignored. If set, the resource type must be one of:
  "vcd_catalog", "vcd_catalog_item", "vcd_catalog_media", "vcd_catalog_vapp_template", "vcd_independent_disk", "vcd_network_direct",
  "vcd_network_isolated", "vcd_network_isolated_v2", "vcd_network_routed", "vcd_network_routed_v2", "vcd_org", "vcd_org_vdc", "vcd_provider_vdc",
  "vcd_storage_profile", "vcd_vapp", "vcd_vapp_vm" or "vcd_vm", which are the resources compatible with metadata_entry.
  Any other resource type will not have effect.
• resource_name: Optional, specifies the name of the object which metadata needs to be ignored. All object types are supported, except for vdcStorageProfile which cannot be filtered by name.
• key_regex: Optional, it is a regular expression that can filter out metadata keys that match.
• value_regex: Optional, it is a regular expression that can filter out metadata keys that match.
• conflict_action: Optional, defines what to do if a conflict exists between a metadata_entry that is managed
  by Terraform, and it matches the criteria defined in the ignore_metadata_changes block, as the metadata will be stored in state but nothing will be done in VCD. If the value is error, when this happens, the Plan will fail. When the value is warn, it will just give a warning but the Plan will continue, and with the none value nothing will be shown. Defaults to error.

Either key_regex or value_regex are required on every ignore_metadata block.

Example

provider "vcd" {
  user                 = "administrator"
  password             = "SuperSecure123"
  auth_type            = "integrated"
  url                  = "https://my-awesome-vcd.com"
  sysorg               = "System"
  org                  = "myOrg"
  allow_unverified_ssl = true
  logging              = true

  # Filters all metadata with key "Environment" or "environment" in all VCD objects with any name.
  ignore_metadata_changes {
    key_regex   = "^[Ee]nvironment$"
  }

  # Filters all metadata with key "NiceMetadataKey" in all VCD objects named "SpecificName".
  ignore_metadata_changes {
    resource_name = "SpecificName"
    key_regex   = "^NiceMetadataKey$"
  }

  # Filters all metadata with values "Yes" in the Organization named "Tatooine".
  ignore_metadata_changes {
    resource_type = "org"
    resource_name = "Tatooine"
    value_regex = "^Yes$"
  }
}

Acceptance tests passed on VCD versions: TBD

[lvirbalas]

I have reviewed the 26 commits done after my last review and it all feels right and further improved!

[Didainius]

2nd approval.
I have manually tested metadata conflict situation and double checked the origin field change.

To reiterate a note of caution about createcalling update. We do not want to do it because of how messy and painful it becomes to deal with it (#901). Yet we still have some places where it happens (and is needed) from a long long time ago.

[adezxc]

Thanks for the feature!