make commands added by installed plugins show in dnf help

I have testet it on a local build

**dnf** without any parameters show the plugin cmd in a separate block
the same does **dnf help**
dnf --help does not show the plugin cmds, because the plugins is not loaded when triggered.

It is hard to make a simple testcase to test it.

It looks like the copr plugin has a get_summery there returns a very long string, there make the output look bad.

all unittest passes after the change.