Only search for Python-2 interpreters

Only search for Python-2 interpreters

Ganeti is not yet ready for Python 3. Therefore, at configure-time,
we effectively have two constraints for the version of Python to
use: ">=2.6" and "<3.0". As, unfortunuately, the AM_PYTHON_PATH
macro only supports minimal-version constraints, we afterwards verify
that we got an interpreter for Python 2. We also tune the search
heuristics to first check binaries that most likely are interpreters
for Python 2. To also work with older versions of the autotools, we
do the "<3.0" check manually and no rely on it already being supported
in the AM_PYTHON_CHECK_VERSION macro.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>