doc: sphinx conf: use env var in sphinx doc version

Changed conf.py to use env var in the version of documentation.
CI can take advantage of this to name documentation using
tag version from git.

Change-Id: If59f800ccd37648a9c0dab6c948b13166a7f4aed
Signed-off-by: Javier B Perez <javier.b.perez.hernandez@intel.com>