Allow setting the Koji tag extra options using the `conf.koji_tag_extra_opts`.
The Koji tag extra options used to be hard-coded and to change them,
we had to release new MBS version.
We do not change them often, but right now fedora-infra is requesting
to use new `mock.new_chroot` option and we need to release new MBS
because of that.
This commit makes such changes easier in the future.