Fix ESG IP Ext Subnet Sel support for IPv6

[jmiguelhp]

Potential solution for this Issue 375

In Terraform NAC, we do not put restriction to vars when IPv4 and IPv6 are supported. nac-validate tool takes care of these syntactic checks, instead.

Validation:

> terraform plan 2>&1 | grep -A 10 fvExternalSubnetSelector
  # module.aci.module.aci_endpoint_security_group["i375terraform/OCP-L3OUT-ANP/OCP-FLOATING-L3OUT-ESG"].aci_rest_managed.fvExternalSubnetSelector["2001:420:207f:f858::0/64"] will be created
  + resource "aci_rest_managed" "fvExternalSubnetSelector" {
      + annotation  = "orchestrator:terraform"
      + class_name  = "fvExternalSubnetSelector"
      + content     = {
          + "descr"  = "OCP-FLOATING-L3OUT External Subnet Selector"
          + "shared" = "no"
        }
      + dn          = "uni/tn-i375terraform/ap-OCP-L3OUT-ANP/esg-OCP-FLOATING-L3OUT-ESG/extsubselector-[2001:420:207f:f858::0/64]"
      + escape_html = true
      + id          = (known after apply)
    }