Revert "Switch to Apache II License", restore GPLv3+

This reverts commit 21d31238dba33e2441a1798e0569495389ab9cb5 and sets the
licence back to GPLv3+ to match Foreman.

puppetdb_foreman is likely a derivative work according to the FSF
interpretation: http://www.gnu.org/licenses/gpl-faq.html#GPLAndPlugins

It interacts with specific Foreman APIs (plugin registration, menus, RBAC),
shares Foreman models (hosts, settings), hooks (e.g. after_build), and only
makes sense when combined with Foreman.

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Waffle aside, I'm doing this just to avert any possible challenge, nothing's come up, but it concerns me that it's incorrect.

Could I ask each contributor to the project to confirm that they're happy to relicense this back to GPLv3+?  (If that's necessary, I'm unsure!)  Comments and question are also very welcome.

- [x] @eLobato
- [x] @mmoll 
- [x] @cyberkov 
- [x] @edestecd 
- [x] @traylenator 
- [x] @hjensas 

Cheers all.