Add support for OMERO_SESSION_DIR environement variable

With this commit, having OMERO_SESSION_DIR set should allow all CLI commands
to default to another sessions directory than the default one.
Suppress the session-dir argument help and expose the envvar
Fix extended help to use OMERO_SESSIONS_DIR
Add unit tests for sessions store directory
Fix OMERO_SESSION_DIR help indentation
Merge remote-tracking branch 'origin/develop' into omero_session_dir

Conflicts:
	components/tools/OmeroPy/src/omero/cli.py
Fix flake8 warnings
Invert order of optional session arguments and environment variables
Merge pull request #3032 from sbesson/omero_session_dir

CLI: add OMERO_SESSION_DIR environment variable