MXS-2313: Add enum to value conversion function

The helper function makes it easier to convert enum values at runtime to
their integer representation. Also changed the configuration processing
code to use the new function.

server/core/internal/config.hh

@@ -255,3 +255,16 @@ std::string generate_config_string(const std::string& instance_name, const MXS_C
  * @return True if the parameter can be modified at runtime
  */
 bool config_can_modify_at_runtime(const char* name);
+
+// Value returned for unknown enumeration values
+constexpr int64_t MXS_UNKNOWN_ENUM_VALUE {-1};
+
+/**
+ * Convert enum name to integer value
+ *
+ * @param key    The enum name to convert
+ * @param values The list of enum values
+ *
+ * @return The enum value or MXS_UNKNOWN_ENUM_VALUE on unknown value
+ */
+int64_t config_enum_to_value(const std::string& key, const MXS_ENUM_VALUE* values);