Add Nova get-password support

Add support to get a encrypted administrative password for a server
through a GET on: /v2.1/{tenant_id}/servers/{server_id}/os-server-password
optionally decrypting the password if a private key is supplied.

The same operation with OpenStack CLI is done with:
  nova get-password <server_id> [private_key.pem]
diff --git a/openstack/compute/v2/servers/urls.go b/openstack/compute/v2/servers/urls.go
index 8998354..d51fcbe 100644
--- a/openstack/compute/v2/servers/urls.go
+++ b/openstack/compute/v2/servers/urls.go
@@ -45,3 +45,7 @@
 func listAddressesByNetworkURL(client *gophercloud.ServiceClient, id, network string) string {
 	return client.ServiceURL("servers", id, "ips", network)
 }
+
+func passwordURL(client *gophercloud.ServiceClient, id string) string {
+	return client.ServiceURL("servers", id, "os-server-password")
+}