Single character wildcard doesn't work in MaxScale

Single character wildcard in the IP address of a grant, i.e. _, does not work. MaxScale confuses it as a hostname and fails to transform it into a network address.

Steps to reproduce:

1. Create a user with a single character wildcard in the host

   CREATE USER 'test'@'192.168.1.11_ IDENTIFIED BY 'test';

   GRANT SELECT ON *.* TO 'test'@'192.168.1.11_';

1. Start MaxScale and inspect the error log

2015-10-20 09:53:14   Error: Failed to obtain address for host 192.168.1.11_, No address associated with hostname