Get default python versions from interpreter

Query the python2/python3 interpreter for it's version to fill in
PYTHON3_VERSION and PYTHON2_VERSION defaults.  This means on a
python3.6 platform such as Fedora 26, we don't need to override the
default.

Change-Id: Id826f275b99b9f397b95e817941019fc503daa1d
2 files changed