Change to modules for build of web documentation

Change to modules for build of web documentation

The modules loaded to build the web documentation with Sphinx have been
incorrect for the minimum version specified by the configuration file.
In particular, the imgmath extension had not been available for
version 1.3 that was indicated as being the minimum version. As
there are no references that I found to any math macros in the files
used to build the docs, I removed the extension to make sure it will
build again. It might be better to have a conditional there, building the
docs without imgmath when using lower versions of sphinx, and having it
active for higher versions.

Changed to require 1.4.1 for now, and added variables that set it
automatically from the information passed to cmake.

Change-Id: Ia329575288e5d622b8e679d76b63759bae54a3b0