debian: Use pybuild to handle OVS Python module.

At present, the OVS Python module is built and installed after
`dh_install`.  This was done to ensure proper linking of the
_json Python extension.

A side effect of this approach is that .install files unexpectedly
do not work, and consequently placement of Python executables
become cumbersome and non-standard.

The preferred approach to this is to make use of pybuild.  In
addition to solving the .install file problem, it also takes care
of building extensions for every requested/supported Python
version, without the need for retrieving and looping over versions
in debian/rules.

We still can't use fully standard Python handling, as we have
interdependencies between C and Python parts of the code base to
build the Python JSON extension.

Signed-off-by: Frode Nordahl <fnordahl@ubuntu.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

debian/python3-openvswitch.install

@@ -1 +1,9 @@
+# At the dh_install stage we need to retain python version specific directory
+# tree to support extensions.
+#
+# dh_python will consolidate into usr/lib/python3/dist-packages retaining
+# version specific shared object files.
+usr/lib/python3* usr/lib/
+
+usr/bin/ovs-flowviz
 usr/share/man/man8/ovs-flowviz.8