Description of problem: As part of an initiative to run Mock in Fedora Toolbox: https://github.com/rpm-software-management/mock/pull/234 we hit the issue that we need to specify rpm macro for DNF. Namely `_netsharedpath`. Right now it is not possible and only way is to mangle HOME and rpmmacros file. RPM itself allows you to specify one-time definition of macros. Both using API and command line. It would be nice if DNF can accept the macros overrides and pass it to RPM. I do not need it on the command line. Allowing to define it in dnf.conf is perfect. Just mind that I may want to define more than one macro.