Add 'google_billing_account' data source (hashicorp#889)

* Add 'google_billing_account' data source.

* Use 'GetResourceNameFromSelfLink'.

* Use 'ConflictsWith' in schema.

* Use pagination for List() API call.

* Add ability to filter by 'open' attribute.

* Don't use 'ForceNew' for data sources.

* Add 'billing_account' argument and make 'name' an output-only attribute.

* Correct error message.

Author: ewbankkit

docs/d/google_billing_account.html.markdown

@@ -0,0 +1,46 @@
+---
+layout: "google"
+page_title: "Google: google_billing_account"
+sidebar_current: "docs-google-datasource-billing-account"
+description: |-
+  Get information about a Google Billing Account.
+---
+
+# google\_billing\_account
+
+Use this data source to get information about a Google Billing Account.
+
+```hcl
+data "google_billing_account" "acct" {
+  display_name = "My Billing Account"
+  open         = true
+}
+
+resource "google_project" "my_project" {
+  name       = "My Project"
+  project_id = "your-project-id"
+  org_id     = "1234567"
+
+  billing_account = "${data.google_billing_account.acct.id}"
+}
+```
+
+## Argument Reference
+
+The arguments of this data source act as filters for querying the available billing accounts.
+The given filters must match exactly one billing account whose data will be exported as attributes.
+The following arguments are supported:
+
+* `billing_account` (Optional) - The name of the billing account in the form `{billing_account_id}` or `billingAccounts/{billing_account_id}`.
+* `display_name` (Optional) - The display name of the billing account.
+* `open` (Optional) - `true` if the billing account is open, `false` if the billing account is closed.
+
+~> **NOTE:** One of `billing_account` or `display_name` must be specified.
+
+## Attributes Reference
+
+The following additional attributes are exported:
+
+* `id` - The billing account ID.
+* `name` - The resource name of the billing account in the form `billingAccounts/{billing_account_id}`.
+* `project_ids` - The IDs of any projects associated with the billing account.

google.erb

@@ -13,6 +13,9 @@
     <li<%= sidebar_current("docs-google-datasource") %>>
     <a href="#">Google Cloud Platform Data Sources</a>
     <ul class="nav nav-visible">
+      <li<%= sidebar_current("docs-google-datasource-billing-account") %>>
+        <a href="/docs/providers/google/d/google_billing_account.html">google_billing_account</a>
+      </li>
       <li<%= sidebar_current("docs-google-datasource-client-config") %>>
         <a href="/docs/providers/google/d/datasource_client_config.html">google_client_config</a>
       </li>
@@ -136,7 +139,7 @@
       </li>
       <li<%= sidebar_current("docs-google-service-account") %>>
         <a href="/docs/providers/google/r/google_service_account.html">google_service_account</a>
-      </li>      
+      </li>
       <li<%= sidebar_current("docs-google-service-account-iam") %>>
         <a href="/docs/providers/google/r/google_service_account_iam.html">google_service_account_iam_binding</a>
       </li>