`bundle exec rake spec_prep` errors out

Following instructions on the OpenStack Docs site for puppet testing [1], running `bundle exec rake spec_prep` errors out with

```
--------------------------------------------------------------------------------
| Install Modules |
--------------------------------------------------------------------------------
+ install_modules_unit
+ '[' -d /home/zuul/src/opendev.org ']'
+ cat /home/jake/work/git/nectar/puppet-magnum/openstack/puppet-openstack-integration/Puppetfile_unit
+ install_all
+ r10k -v DEBUG puppetfile install --puppetfile /home/jake/work/git/nectar/puppet-magnum/openstack/puppet-openstack-integration/Puppetfile --moduledir /home/jake/work/git/nectar/puppet-magnum/spec/fixtures/modules
The git source https://opendev.org/openstack/puppet-openstack_spec_helper is not yet checked out. Please run `bundle install` before trying to start your application
```

I've traced the code to this commit [2]. I've asked about this in the IRC channel, the workaround is to do `export ZUUL_BRANCH=master`.

Should this be fixed up, or should the documents reflect this workardoun?

By the way, this also affects other puppet modules that uses the same Gemfile, like puppet-nova.

[1]: https://docs.openstack.org/puppet-openstack-guide/latest/contributor/testing.html#tiny-trick-for-rspec
[2]: https://opendev.org/openstack/puppet-magnum/commit/6710dcc476f744c3e845c4b0f029d57fc0c1e2bb