Add USES azurepy: Shared namespace for py-azure-* ports

USES=azurepy provides common files that are shared among all
py-azure-* ports:

azure/__init__.py
azure/cli/__init__.py
azure/cli/command_modules/__init__.py
azure/mgmt/__init__.py
azure/mgmt/datalake/__init__.py

Reviewed by: antoine, mat
Approved by: portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D18573