Foreman » Installer

Add simple acceptance tests

Run tests one by one with:

```
PUPPET_INSTALL_TYPE=agent \
BEAKER_debug=yes \
BEAKER_set=centos-7-x64 \
BEAKER_destroy=no \
bundle exec rspec spec/acceptance/puppetserver_upgrade_2_6_0_to_2_7_2_spec.rb

PUPPET_INSTALL_TYPE=agent \
BEAKER_provision=no \
BEAKER_debug=yes \
BEAKER_set=centos-7-x64 \
BEAKER_destroy=no \
bundle exec rspec spec/acceptance/puppetserver_latest_spec.rb
```

the second call can also includes `BEAKER_provision=no` so the
existing VM gets re-used instead of fully re-created.