Add containers-only option to configtest

[knqyf263]

What did you implement:

I'd like to scan only containers.
scan subcommand has -containers-only option.
#190

However, configtest doesn't have -containers-only option.
Then, an error about a host server occur.

How did you implement it:

I added -containers-only option to configtest.

How can we verify it:

$ vuls configtest -containers-only

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?: YES
Is it a breaking change?: NO

[kotakanbe]

Many Thanks :)