In #7800, Nomad would automatically generate a port label for service
checks making use of the expose feature, if the port was not already
set. This change assumed the group network would be correctly defined
(as is checked in a validation hook later). If the group network was
not definied, a panic would occur on job submisssion. This change
re-uses the group network validation helper to make sure the network
is correctly definied before adding ports to it.
Fixes#8875