test: enable parallel run for python test suites

Fixed issues:

- box-py/iproto.test.py
  1) Fixed receive_response() to wait for whole response.
  2) Clean up _cluster space.

- replication-py/cluster.test.py
  1) Clean up _cluster space.

- replication-py/multi.test.py
  1) Removed vclock checking because it fails if previous test make some
     DML and vclock was incremented. Looks like it was used for debug
     and is not part of this test case.
  2) Fixed typo in 'Synchronize' block.

The following test sequences did fail due to unexpected IDs in _cluster
space:

- [box-py/iproto.test.py, null]
- [box-py/bootstrap.test.py, null]

- [replication-py/cluster.test.py, null]
- [replication-py/multi.test.py, null]

Part of #3232

(cherry picked from commit 4ea7d729)