Add vault client initialization check

Make a request when creating the client to see if the Vault is unsealed and available for operation.

@yordan.kinkov @valerii.kalashnikov

cmd/signer/main.go

@@ -50,9 +50,9 @@ func main() {
 
 	httpClient := httpClient()
 
-	vault, err := vault.New(cfg.Vault.Addr, cfg.Vault.Token, httpClient)
+	vault, err := vault.New(cfg.Vault.Addr, cfg.Vault.Token, true, httpClient)
 	if err != nil {
-		logger.Fatal("cannot create vault client", zap.Error(err))
+		logger.Fatal("cannot initialize vault client", zap.Error(err))
 	}
 
 	// create services