Mark identity_source_id as Computed on policy role binding (#2302)

NSX can populate this field even when it's left unset in config,
causing spurious drift. Mark it Computed so Terraform accepts the
server-assigned value.

Partial port of #1959 ("Revise LDAP tests to pass with OpenLDAP
server"): only this production schema fix is applicable here, the
rest of that PR revises LDAP acceptance tests to target OpenLDAP
infrastructure specifics unrelated to this branch.

Author: ksamoray

nsxt/resource_nsxt_policy_role_binding.go

@@ -66,6 +66,7 @@ func resourceNsxtPolicyUserManagementRoleBinding() *schema.Resource {
 				Type:        schema.TypeString,
 				Description: "ID of the external identity source",
 				Optional:    true,
+				Computed:    true,
 			},
 			"identity_source_type": {
 				Type:         schema.TypeString,