Add vault_policy_document data source

[zunkree]

Add following data source to manage Vault policies, fix for #64

• vault_policy_document: render HCL with Vault policy

To test this PR:

provider "vault" {}

 data "vault_policy_document" "example" {
  rule {
    path         = "secret/*"
    capabilities = ["create", "read", "update", "delete", "list"]
    description  = "allow all on secrets"
  }
}

resource "vault_policy" "example" {
  name   = "example_policy"
  policy = "${data.vault_policy_document.hcl}"
} 

[zunkree]

Also, is it possible in terraform to make argument like:

 data "vault_policy_document" "example" {
  path "secret/*" {
    capabilities = ["create", "read", "update", "delete", "list"]
    description  = "allow all on secrets"
  }
}

[zunkree]

hey guys, any feedback about that PR?

[tyrannosaurus-becks]

@zunkree this is fantastic! Thank you!