CLI: add OMERO_SESSION_DIR environment variable

Addresses https://www.openmicroscopy.org/community/viewtopic.php?f=6&t=7626

This PR:
- adds support for an `OMERO_SESSION_DIR` environment variable in the CLI. This variable simply sets the  directory  where sessions are stored, defaulting to `~/omero/sessions`.
- deprecates the `session-dir` argument using `argparse.SUPPRESS`
- adds a simple sessions unit test

With this changes, it should be possible to run all CLI commands in a custom sessions directory by setting this environment variable, e.g.

```
$ export OMERO_SESSION_DIR=/tmp
$ bin/omero hql -C ...
```