Fix to MXS-288: https://mariadb.atlassian.net/browse/MXS-288 Added MySQL 5.7 authentication

The change to the mysql.user table in MySQL 5.7 caused MaxScale to stop
working with it. This commit adds functionality that checks which version of
the user data query should be made. It also moves common code related to
server version strings to server.c

server/include/server.h

@@ -212,4 +212,5 @@ extern void	server_update_address(SERVER *, char *);
 extern void	server_update_port(SERVER *,  unsigned short);
 extern RESULTSET	*serverGetList();
 extern unsigned int server_map_status(char *str);
+extern bool server_set_version_string(SERVER* server, const char* string);
 #endif