Python integration test fixes, pytest and target (rebased onto develop)

This is the same as gh-1409 and gh-1493 but rebased onto develop.

----

Enable all pytests (`OmeroPy/test`) including gatewaytests via the "integration" target. This means all of the following should now run Python:

```
 ome.git$ ./build.py test-integration
 ome.git$ cd components/tools/OmeroPy
 OmeroPy$ ../../../build.py integration
 OmeroPy$ ../../../build.py python-integration
```

For further testing details see #1409