Current BuildConfigFromFlags() seems to print a log like below when
config path isn't specified:
```
W0120 15:30:06.196820 13323 client_config.go:359] error creating
inClusterConfig, falling back to default config: %vunable to load
in-cluster configuration, KUBERNETES_SERVICE_HOST and
KUBERNETES_SERVICE_PORT must be defined
```
This commit removes the needless "%v"
a33f459c39