Config modules are able to declare distros that they were verified
to run on by setting 'distros' as a list in the config module.
Previously, if a module was configured to run and the running distro was not
listed as supported, it would run anyway, and a warning would be written.
Now, we change the behavior to skip those modules.
The distro (or user) can specify that a given list of modules should run anyway
by declaring the 'unverified_modules' config variable.
run_once modules will be run without this filter (ie, expecting that the user
explicitly wanted to run it).
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