Additional avocado plugin package required upon upgrading to avocado 51.0

[mmathesius]

The following statement can be found in the avocado 51.0 release notes:

The yaml-to-mux varianter implementation (the only one at this point) is now an optional plugin. Basically, this means that users deploying this (and later) version of Avocado, should also explicitly install it.

Although it may not immediately be obvious from the above statement, anything that runs the avocado command using the --mux-inject command line option (such as run-checkmmd.sh and runtask.yml) will fail when using avocado 51.0 or later unless they also install the yaml-to-mux plugin.

The python2-avocado-51.0-1 and corresponding python2-avocado-plugins-varianter-yaml-to-mux-51.0-1 packages are currently available in the Fedora updates-testing repositories.