only set randompasswd in graphics template if it's true (#535)

Small fix in the template schemas for older OpenNebula versions. 

Older OpenNebula versions (like 5.12) interpret the `RANDOM_PASSWORD`
as truthy if its value is not empty and therefore try to generate a
password, which when using VNC will not be truncated due to another
bug that's already been fixed in the latest release.

opennebula/shared_schemas.go

@@ -652,13 +652,14 @@ func addGraphic(tpl *vm.Template, graphics []interface{}) {
 				case "passwd":
 					tpl.AddIOGraphic(vmk.Passwd, v.(string))
 				case "random_passwd":
-					// Convert bool to string
-					tpl.AddIOGraphic(vmk.RandomPassword, map[bool]string{true: "YES", false: "NO"}[v.(bool)])
+					// only set random_passwd if it's set to true -- older OpenNebula versions will consider any
+					// non-zero string as a yes
+					if v.(bool) {
+						tpl.AddIOGraphic(vmk.RandomPassword, "YES")
+					}
 				}
-
 			}
 		}
-
 	}
 }