Related to #3681
If a user specifies an invalid port *label* when using
address_mode=driver they'll get an error message about the label being
an invalid number which is very confusing.
I also added a bunch of testing around Service.AddressMode validation
since I was concerned by the linked issue that there were cases I was
missing. Unfortunately when address_mode=driver is used there's only so
much validation that can be done as structs/structs.go validation never
peeks into the driver config which would be needed to verify the port
labels/map.