Add Security Tags resource

[mikeletux]

There is not issue associated with this PR.

Description

This PR brings to the Terraform provider a new resource to create security tags on VCD.

Detailed description

With this PR, the Terraform provider for VCD introduces the possibility of creating security tags via new vcd_security_tag resource.
With this resource, a new security tag can be created and can be associated to several VMs. An example would be:

resource "vcd_security_tag" "my_tag" {
  name = "test-tag"
  vm_ids = [vcd_vm.my-vm-one.id, vcd_vm.my-vm-two.id]
}

Where name is the security tag name and vm_ids a list of VM IDs that this security tag is going to be applied to.

Alongside this resource it comes acceptance tests as well as Terraform documentation on how to use this new resource.

[vbauzys]

I will play manually to check some things.

[Didainius]

Pretty much last comments - should be good to go after fixes.

[Didainius]

Thanks!

[adambarreiro]

Just a nit-like suggestion, to rename the PR to something more specific like "Add Security Tags resource"

[adambarreiro]

Great work 💪