Fix configuration of node name for Jenkins agents (sysadmin#17846)

Our current Jenkins config expects agents hosted in Lizard to be called
by their hostname (eg. isotesterN) and agents hosted elsewhere to be
called by their FQDNs (isotesterN.domain).

Despite that, such config was broken but a problem didn't manifest
because there were no non-Lizard agents in the Lizard Jenkins until now.
With the introduction of the new CI server Iguana and it's VMs, we need
to fix how nodes names are calculated and passed around.