python: Implement write support in Python IDL for OVSDB.

python: Implement write support in Python IDL for OVSDB.

Until now, the Python bindings for OVSDB have not supported writing to the
database.  Instead, writes had to be done with "ovs-vsctl" subprocesses.
This commit adds write support and brings the Python bindings in line with
the C bindings.

This commit deletes the Python-specific IDL tests in favor of using the
same tests as the C version of the IDL, which now pass with both
implementations.

This commit updates the two users of the Python IDL to use the new write
support.  I tested this updates only by writing unit tests for them,
which appear in upcoming commits.