raft: fix logger Panic variadic parameter
"# github.com/coreos/etcd/raft
raft/logger.go:117: missing ... in args forwarded to print-like function"
New parameter check got added the golang to check the function parameter
https://github.com/golang/go/commit/c00603607511701ecc9f56fd82ac528ecf6b8fc6#diff-8fa5b0d6191706747ef5773f895781c9