Check yum plugin enabled

[knqyf263]

What did you implement:

Closes #284

How did you implement it:

Check if the following plugins are enabled.

• CentOS
  • changelog
• RHEL
  • security

I use yum --help and parse the first line.
e.g. Loaded plugins: changelog, fastestmirror, security

How can we verify it:

Use prepare command.

Todos:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: NO (I haven't test on RHEL yet.)
Is it a breaking change?: NO

[kotakanbe]

Thanks for sending P/R.
But the prepare subcommand has been deprecated.
I decided to use configtest to check if yum-plugin-security is enabled. #375

Sorry I could not merge this P/R to master though you sent a pull request.