feat: relax tf version to allow 0.13

[bharathkkb]

• relax version constraints
• update inspec tests to 1.8
• update pf

[bharathkkb]

@rjerrems tested this with TF0.13 image in https://github.com/terraform-google-modules/terraform-google-bootstrap/runs/1290489785

[rjerrems]

Thanks @bharathkkb - do you know why the integration tests are failing on the latest commit? I re-triggers and it seems to fail with the same inspec error:

     ×  Control Source Code Error /workspace/test/integration/cloudbuild_enabled/controls/gcp.rb:61 
     undefined method `split' for nil:NilClass

Is this because the inspec test have been upgraded perhaps?

[bharathkkb]

@rjerrems we updated the gemfile in a later 0.13 image. Those changes were backported the 0.12.2 , so I have used that image now. A side effect is that I also had to regen docs with the new tfdocs 8b20542

[rjerrems]

LGTM