Refactored logic for `PYTHON` and `PYTHON_VERSION` in `configure.ac`.
Refactored logic for `PYTHON` and `PYTHON_VERSION` in `configure.ac`.

This will facilitate the introduction of `PYTHON_3` and
`PYTHON_3_VERSION` to build the CLI in a future commit.

Review: https://reviews.apache.org/r/67413/

reneploetz/mesos,kaysoky/mesos,chhsia0/mesos,kaysoky/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,Gilbert88/mesos,jpeach/mesos,chhsia0/mesos,gsantovena/mesos,Gilbert88/mesos,Gilbert88/mesos,kaysoky/mesos,gsantovena/mesos,gsantovena/mesos,andschwa/mesos,gsantovena/mesos,kaysoky/mesos,andschwa/mesos,chhsia0/mesos,jpeach/mesos,dforsyth/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,dforsyth/mesos,reneploetz/mesos,andschwa/mesos,jpeach/mesos,chhsia0/mesos,abudnik/mesos,andschwa/mesos,gsantovena/mesos,abudnik/mesos,dforsyth/mesos,Gilbert88/mesos,andschwa/mesos,jpeach/mesos,Gilbert88/mesos,Gilbert88/mesos,gsantovena/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,reneploetz/mesos,dforsyth/mesos,abudnik/mesos,gsantovena/mesos,abudnik/mesos,Gilbert88/mesos,chhsia0/mesos,dforsyth/mesos,kaysoky/mesos,chhsia0/mesos,jpeach/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,kaysoky/mesos,andschwa/mesos,chhsia0/mesos,andschwa/mesos