MXS-2304 Add contains() to test if a parameter exists

Replaces uses of config_get_param() in modules either with contains()
or get_string(). The config_get_param() is moved to internal headers,
as it allows seeing inside a config setting.

include/maxscale/config.hh

@@ -318,6 +318,14 @@ public:
      */
     SERVER* get_server(const std::string& key) const;
 
+    /**
+     * Check if a key exists.
+     *
+     * @param key Parameter name
+     * @return True if key was found
+     */
+    bool contains(const std::string& key) const;
+
     char*                    name;          /**< The name of the parameter */
     char*                    value;         /**< The value of the parameter */
     MXS_CONFIG_PARAMETER*    next;          /**< Next pointer in the linked list */
@@ -396,15 +404,6 @@ struct MXS_CONFIG
  */
 MXS_CONFIG* config_get_global_options();
 
-/**
- * @brief Get a configuration parameter
- *
- * @param params List of parameters
- * @param name Name of parameter to get
- * @return The parameter or NULL if the parameter was not found
- */
-MXS_CONFIG_PARAMETER* config_get_param(MXS_CONFIG_PARAMETER* params, const char* name);
-
 /**
  * @brief Helper function for checking SSL parameters
  *