configure: Also check for Python 2.x installed as python2.7.

On ArchLinux "python" points to 3.x version and the right
2.x python binary is "python2.7".

Signed-off-by: Dmitry V. Krivenok <krivenok.dmitry@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>