Fix offline builds

pip 19 added support for something called `pyproject.toml`. Several packages have been setting the option `build-backend = "setuptools.build_meta”` (bcrypt, attrs, jaraco) which seems to be the root of the problem when building from source. Until the community sorts this out I’m inclined to avoid pip 19.

requirements/requirements_ansible.txt

@@ -122,5 +122,5 @@ wheel==0.30.0             # via azure-cli-core
 xmltodict==0.12.0         # via pywinrm
 
 # The following packages are considered to be unsafe in a requirements file:
-pip==19.1.1
+pip==18.1
 setuptools==41.0.1