Switch puppet-tripleo to use puppet-openstack_spec_helper

Switch puppet-tripleo to use puppet-openstack_spec_helper

Align puppet-tripleo with other Puppet OpenStack modules to use
puppet-openstack_spec_helper.

Here are the benefits:

- Allow to use Depends-On between Puppet OpenStack modules and
  puppet-tripleo, and unit tests will work correctly with dependencies.
- Use the Puppet OpenStack gemspec file that handle gems dependencies to
  test the module.
- Allow to re-use ruby helper that Puppet OpenStack modules have in unit
  tests, to avoid duplicated code.
- Don't manage .fixtures.yml file, it will be generated by Puppet
  OpenStack tooling. TripleO dependencies will live in Puppetfile_extras.

Change-Id: Ic66e9f872c57545327a9fb4b8ae86fbf0abbd8be