Add SKIP_GENERATE_AUTHORS option to setup.py
Add SKIP_GENERATE_AUTHORS option to setup.py

Updates the generate_authors function in setup.py so that
it will skip generation of the authors file if the
SKIP_GENERATE_AUTHORS env variable is set.

Authors doesn't currently take that long to generate but having
the option to explicitly skip it (regardless of whether .git is there)
seems like a good addition.

Change-Id: Ie18064d2450cecc86f18d0b10e1ce473253d1a51

varunarya10/pbr,emonty/oslo-packaging,JioCloud/pbr,redhat-openstack/pbr,citrix-openstack-build/pbr,emonty/oslo-packaging,citrix-openstack-build/pbr,openstack-dev/pbr,citrix-openstack-build/pbr,varunarya10/pbr,JioCloud/pbr,WiFast/pbr,redhat-openstack/pbr,WiFast/pbr,openstack-dev/pbr,klmitch/pbr,klmitch/pbr,openstack-dev/pbr