Deprecate name_prefix (hashicorp#1035)

* deprecate name_prefix

* make name_prefix computed and add migration instructions

Author: danawillow

docs/r/compute_instance_template.html.markdown

@@ -125,7 +125,7 @@ The following arguments are supported:
 * `name` - (Optional) The name of the instance template. If you leave
   this blank, Terraform will auto-generate a unique name.
 
-* `name_prefix` - (Optional) Creates a unique name beginning with the specified
+* `name_prefix` - (Deprecated, Optional) Creates a unique name beginning with the specified
   prefix. Conflicts with `name`.
 
 * `can_ip_forward` - (Optional) Whether to allow sending and receiving of

docs/r/compute_ssl_certificate.html.markdown

@@ -70,7 +70,7 @@ The following arguments are supported:
 * `name` - (Optional) A unique name for the SSL certificate. If you leave
   this blank, Terraform will auto-generate a unique name.
 
-* `name_prefix` - (Optional) Creates a unique name beginning with the specified
+* `name_prefix` - (Deprecated, Optional) Creates a unique name beginning with the specified
   prefix. Conflicts with `name`.
 
 * `description` - (Optional) An optional description of this resource.

docs/r/container_node_pool.html.markdown

@@ -109,7 +109,7 @@ resource "google_container_cluster" "primary" {
 * `name` - (Optional) The name of the node pool. If left blank, Terraform will
     auto-generate a unique name.
 
-* `name_prefix` - (Optional) Creates a unique name for the node pool beginning
+* `name_prefix` - (Deprecated, Optional) Creates a unique name for the node pool beginning
     with the specified prefix. Conflicts with `name`.
 
 * `node_config` - (Optional) The node configuration of the pool. See