pfe-variant doesn't get passed into dynamic pfe-tab-panels

[mwcz]

The quickest way to demo the bug is this recording:

To repro:

1. run npm start
2. in browser, click pfe-content-set
3. scroll to the bottom
4. click the Add Header & Panel button
5. go back and forth between the two tabs

The new tab doesn't get the pfe-variant so the content doesn't have the same vertical spacing as the starting tab. Maybe a better way to describe the issue is that dynamically added tabs don't get pfe-variant.

I noticed this while looking at PR #406 but I'm not sure if that PR caused the bug, or just revealed it.

[kylebuch8]

This has been fixed.