Extract cert storer methods from AuthDB

To be able to extend the AuthDB with methods that also extend the
provisioner we need to either create a new method or to split the
interface. This change splits the interface so we can have a cleaner
implementation.

db/db.go

@@ -50,14 +50,19 @@ type AuthDB interface {
 	Revoke(rci *RevokedCertificateInfo) error
 	RevokeSSH(rci *RevokedCertificateInfo) error
 	GetCertificate(serialNumber string) (*x509.Certificate, error)
-	StoreCertificate(crt *x509.Certificate) error
 	UseToken(id, tok string) (bool, error)
 	IsSSHHost(name string) (bool, error)
-	StoreSSHCertificate(crt *ssh.Certificate) error
 	GetSSHHostPrincipals() ([]string, error)
 	Shutdown() error
 }
 
+// CertificateStorer is an extension of AuthDB that allows to store
+// certificates.
+type CertificateStorer interface {
+	StoreCertificate(crt *x509.Certificate) error
+	StoreSSHCertificate(crt *ssh.Certificate) error
+}
+
 // DB is a wrapper over the nosql.DB interface.
 type DB struct {
 	nosql.DB

db/simple.go

@@ -20,7 +20,7 @@ type SimpleDB struct {
 	usedTokens *sync.Map
 }
 
-func newSimpleDB(c *Config) (AuthDB, error) {
+func newSimpleDB(c *Config) (*SimpleDB, error) {
 	db := &SimpleDB{}
 	db.usedTokens = new(sync.Map)
 	return db, nil