teuthology.run: conditionally add fake_ctx to teuthology.config.config

The intent here is to get the 'interactive-on-error' config value
passed down to contextutil.nested(), but it is recognized that we need
a way to share global configuration; for now, since nested() really
needs the entire ctx, we can get it out of fake_ctx.config, but there
needs to be a cleaner way as FakeNamespace is phased out.

Fixes: #10260
Signed-off-by: Dan Mick <dan.mick@redhat.com>