MXS-2483: Make some SSLContext methods const

The serialization and opening of a new SSL session can be const.

include/maxscale/ssl.hh

@@ -81,12 +81,12 @@ public:
      *
      * @return SSLContext as a INI file section
      */
-    std::string serialize();
+    std::string serialize() const;
 
     /**
      * Opens a new OpenSSL session for this configuration context
      */
-    SSL* open()
+    SSL* open() const
     {
         return SSL_new(m_ctx);
     }

server/core/ssl.cc

@@ -236,7 +236,7 @@ SSLContext::SSLContext(const std::string& key, const std::string& cert, const st
 {
 }
 
-std::string SSLContext::serialize()
+std::string SSLContext::serialize() const
 {
     std::stringstream ss;
     ss << "ssl=required\n";