prodvider: use SANs in service certificates

This fixes compatibility with prodaccess tools built with Go 1.15, which
introduced 'X.509 CommonName deprecation' [1].

[1] - https://golang.org/doc/go1.15#commonname

Change-Id: I228cde3e5651a3e36f527783f2ccb4a2f6b7a8e3
diff --git a/cluster/prodvider/certs.go b/cluster/prodvider/certs.go
index bed0e48..309af1f 100644
--- a/cluster/prodvider/certs.go
+++ b/cluster/prodvider/certs.go
@@ -57,6 +57,7 @@
 				OU: signerCert.Subject.OrganizationalUnit[0],
 			},
 		},
+		Hosts: []string{flagProdviderCN},
 	}
 
 	g := &csr.Generator{
@@ -68,7 +69,7 @@
 
 func (p *prodvider) makeSelfCertificate(csr []byte) ([]byte, error) {
 	req := signer.SignRequest{
-		Hosts:   []string{},
+		Hosts:   []string{flagProdviderCN},
 		Request: string(csr),
 		Profile: "server",
 	}