Replace update_strategy with a rolling update_policy

[lbordowitz]

When using version v0.2.0 of the vault-cluster module, I get the following WARNING

Warning: "update_strategy": [DEPRECATED] This field will be replaced by `update_policy` in 3.0.0

  on .terraform/modules/vault_cluster/modules/vault-cluster/main.tf line 54, in resource "google_compute_region_instance_group_manager" "vault":
  54: resource "google_compute_region_instance_group_manager" "vault" {

In addition to the tech-debt paid off by replacing a deprecated field, this also supports rolling update. As understand, this feature was much sought after.

[robmorgan]

Hi @lbordowitz, thanks for posting this issue. I'll discuss the priority with the team and get back to you. In the meantime you're more than welcome to submit a PR.