omit empty AdminPass in 'Rebuild' req body (#267)

diff --git a/openstack/compute/v2/servers/requests.go b/openstack/compute/v2/servers/requests.go
index c79a6e6..9618637 100644
--- a/openstack/compute/v2/servers/requests.go
+++ b/openstack/compute/v2/servers/requests.go
@@ -401,11 +401,10 @@
 // operation
 type RebuildOpts struct {
 	// The server's admin password
-	AdminPass string `json:"adminPass" required:"true"`
+	AdminPass string `json:"adminPass,omitempty"`
 	// The ID of the image you want your server to be provisioned on
 	ImageID   string `json:"imageRef"`
 	ImageName string `json:"-"`
-	//ImageName string `json:"-"`
 	// Name to set the server to
 	Name string `json:"name,omitempty"`
 	// AccessIPv4 [optional] provides a new IPv4 address for the instance.