Allow for inclusion of authors and changelog when building docs
Allow for inclusion of authors and changelog when building docs

When git is available and changelog and author building has not
been turned off try to build these two files so that they can be
included in any subsequent documentation.

Change-Id: I2fc7ade645e3ff1a67e30bd4d35676b5e10916af

varunarya10/pbr,JioCloud/pbr,klmitch/pbr,WiFast/pbr,openstack-dev/pbr,varunarya10/pbr,openstack-dev/pbr,klmitch/pbr,WiFast/pbr,openstack-dev/pbr,JioCloud/pbr