As noted by stephenfin, Sphinx 1.6 provides its own code to build doc with
multiple builders. The one provided by pbr so far for Sphinx < 1.6 is not even
compatible with 1.6. This patch fixes that by running the native Sphinx code
for Sphinx > 1.6 and falling back to the old code for older Sphinx versions.
Closes-Bug: #1691129
Change-Id: I5224235b1056a248b246c54e2d99eea94d53c4eb