debian: Make python-openvswitch packaging work with squeeze dh_python2.

debian: Make python-openvswitch packaging work with squeeze dh_python2.

The dh_python2 helper in Debian squeeze has a limitation that is not
mentioned anywhere, as far as I can tell: Python files must be in
/usr/lib/python#.#/site-packages to be installed.  The version in Debian
wheezy does not have the same limitation.

This meant that building the Debian packages on squeeze silently produced
a broken python-openvswitch package, whereas building the same thing on
wheezy built a working package.

This fixes the problem by putting the .py files where squeeze expects them.
It works on wheezy too.

Bug #7510.
Reported-by: Michael Hu <mhu@nicira.com>
Tested-by: Simon Horman <horms@verge.net.au>