rpm: include a warning message for network configuration on...

rpm: include a warning message for network configuration on /etc/sysconfig/network-scripts directory

NetworkManager now does not support network configuration through
ifcfg files by default anymore, it is provided in a separated
package:
https://fedoramagazine.org/converting-networkmanager-from-ifcfg-to-keyfiles/

This commits include a file in rpm packages located in ifcfg scripts
directory, /etc/sysconfig/network-scripts/, to inform the user of
the new location of network configuration files.

https://bugzilla.redhat.com/show_bug.cgi?id=2074020
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