contrib/rpm: add "00-server-dhcp-client-id.conf"
While this is packaged also in "NetworkManager-config-server.rpm" sub-package, it's not in "00-server.conf" file. The reason is that a convenient way to disable configuration from "/usr/lib/NetworkManager/conf.d", is by putting a (possibly empty) file into /etc directory with the same name. If the sub-package only provides one "00-server.conf" file, this is no longer possible at a granular level.