The nethserver-diagtools package is now available from nethserver-updates repository!
It will be shipped as a default package, starting from the next ISO release. To install it on 7.3.1611 run
yum update @nethserver-iso
Otherwise run
yum localinstall http://packages.nethserver.org/nethserver/7.3.1611/updates/x86_64/Packages/nethserver-diagtools-1.0.0-1.ns7.noarch.rpm