Define master_doc in sphinx-build configuration
Define master_doc in sphinx-build configuration

- Fixes #2159
- Fixes the build when using sphinx-build version 1.8.5

Signed-off-by: Christopher M. Cantalupo <9cf3ecca60a3dd7112ab0e7250bedb8a752e3958@intel.com>

cmcantalupo/geopm,geopm/geopm,geopm/geopm,cmcantalupo/geopm,geopm/geopm,cmcantalupo/geopm,geopm/geopm,geopm/geopm,cmcantalupo/geopm,cmcantalupo/geopm