Fixed two small bugs in GCENodeDriver.ex_create_firewall().
The first issue was that a direction check was being applied to determine whether allowed or denied would be passed to the GCE API call. Both ingress and egress rules can have allowed or denied blocks so this shouldn't be conditional on the direction.
The second issue was that source_ranges was automatically set to [0.0.0.0/0] if no other source filters were applied. Since source filters are invalid for egress rules, I added a check for the INGRESS direction prior to setting a default source range.