Minogrpc certificate: hostname

[Gilthoniel]

@gnarula you may want to address this for the student project.

Basically its generating a certificate for 127.0.0.1 but it should use the address hostname/IP.

func (o *overlay) makeCertificate() error {
	tmpl := &x509.Certificate{
		SerialNumber: big.NewInt(1),
		IPAddresses:  []net.IP{net.ParseIP("127.0.0.1")},

It should instead check if its an IP or a hostname and fill the appropriate field of the certificate.