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
Update develop submodules

Repository: openmicroscopy/openmicroscopy
Already up-to-date.

Repository: openmicroscopy/bioformats
Updating 0819d22..46f4115
Previously merged:
  # PR 1323 melissalinkert 'Add bit-level operations to RandomAccess{Input, Output}Stream'
  # PR 1342 joshmoore 'Catch all exceptions on loadMemo'
  # PR 1340 joshmoore 'kryo: update to 2.24.0 with shading changes (see #10852)'
  # PR 1331 rleigh-dundee 'C++: Add FormatWriter interface'
  # PR 1335 melissalinkert 'ND2: prevent NumberFormatExceptions when parsing text data (rebased onto develop)'
  # PR 1324 melissalinkert 'Deltavision: conditionally ignore stage position values of 0'
  # PR 1317 melissalinkert 'Gatan: fix physical dimension parsing when the Z size is missing'

Repository: ome/scripts
Already up-to-date.

Repository: openmicroscopy/ome-documentation
Updating 01a91b2..9dbefc4
Previously merged:
  # PR 986 sbesson 'Document usage of new OMERO_SESSION_DIR envvar for the CLI'
  # PR 987 snoopycrimecop 'Update auto-generated properties'
  # PR 984 aleksandra-tarkowska 'adding support for django directives'

Generated by OMERO-5.1-latest-submods#199 (http://ci.openmicroscopy.org/job/OMERO-5.1-latest-submods/199/)