[v15] agent-pool-terraform: Pick random characters from a pool acceptable to Teleport backends (#42763)

* agent-pool-terraform: Pick random characters from a pool acceptable to Teleport backends

* Also add version to provision token resource

Author: webvictim

examples/agent-pool-terraform/teleport/token.tf

@@ -1,10 +1,12 @@
 resource "random_string" "token" {
-  count  = var.agent_count
-  length = 32
+  count            = var.agent_count
+  length           = 32
+  override_special = "-.+"
 }
 
 resource "teleport_provision_token" "agent" {
-  count = var.agent_count
+  count   = var.agent_count
+  version = "v2"
   spec = {
     roles = var.agent_roles
   }