Add product type private_ssl_flex (#8)

Equals `Private SSL OV` in certcentral UI

Author: hoffimar

order.go

@@ -22,6 +22,7 @@ var OrderTypes = struct {
 	SSLPlus,
 	SSLMultiDomain,
 	SSLWildcard,
+	PrivateSSLOV,
 	PrivateSSLPlus,
 	PrivateSSLWildcard,
 	SecureSiteProSSL,
@@ -32,6 +33,7 @@ var OrderTypes = struct {
 	"ssl_plus",
 	"ssl_multi_domain",
 	"ssl_wildcard",
+	"private_ssl_flex",
 	"private_ssl_plus",
 	"private_ssl_wildcard",
 	"ssl_securesite_pro",