MXS-2861: Add ssl_cipher to servers and listeners

This allows the set of TLSv1.2 ciphers to be selected which is required to
fulfill certain regulatory requirements.

server/core/ssl.cc

@@ -201,6 +201,7 @@ SSLConfig::SSLConfig(const MXS_CONFIG_PARAMETER& params)
     , version((ssl_method_type_t)params.get_enum(CN_SSL_VERSION, ssl_version_values))
     , verify_depth(params.get_integer(CN_SSL_CERT_VERIFY_DEPTH))
     , verify_peer(params.get_bool(CN_SSL_VERIFY_PEER_CERTIFICATE))
+    , cipher(params.get_string(CN_SSL_CIPHER))
 {
 }
 
@@ -384,6 +385,15 @@ bool SSLContext::init()
     /* Set the verification depth */
     SSL_CTX_set_verify_depth(m_ctx, m_cfg.verify_depth);
 
+    if (!m_cfg.cipher.empty())
+    {
+        if (SSL_CTX_set_cipher_list(m_ctx, m_cfg.cipher.c_str()) == 0)
+        {
+            MXS_ERROR("Could not set cipher list '%s': %s", m_cfg.cipher.c_str(), get_ssl_errors());
+            return false;
+        }
+    }
+
     return true;
 }