Fix CLI import as with -g argument

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/12288

With this PR, the CLI import as functionality should now properly work in multi-group, i.e.
```
bin/omero import --sudo root -u username -s servername -g groupname file
```
should import file as user `user` in group `groupname`.

Also check the integration tests previously marked as `xfail` now pass. 

Note the first integration test `testImportMultiGroup` was wrongly marked as xfail. Reason of failure was the connection arguments used for the test i.e. using session key was taking precedence over group argument. This should also be fixed via the restructuring of the `set_conn_args` method.