Add support for patch to vault_policy_document

[benashz]

Update vault_policy_document data source to support the patch capability.

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Closes #1227

Release note for CHANGELOG:

Output from acceptance testing:

$ make dev testacc TESTARGS='-v -test.run TestDataSourcePol'            
==> Checking that code complies with gofmt requirements...
go build -o terraform-provider-vault
mv terraform-provider-vault ~/.terraform.d/plugins/
TF_ACC=1 go test $(go list ./...) -v -v -test.run TestDataSourcePol -timeout 20m

=== RUN   TestDataSourcePolicyDocument
--- PASS: TestDataSourcePolicyDocument (1.94s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     4.881s

...