python3 support

[joshmoore]

Propose release to Docker Hub as 5.6.0-m1 but not latest.

see: ome/omero-server-docker#33

[joshmoore]

Currently failing locally with:

TASK [ome.omero_web : omero web | systemd service] *****************************
skipping: [localhost]
[WARNING]: flush_handlers task does not support when conditional

RUNNING HANDLER [ome.omero_common : restart omero-web] *************************
fatal: [localhost]: FAILED! => {"changed": false, "msg": "Could not find the requested service omero-web: host"}

[joshmoore]

Pushed 5.6.0-m1 tag from this pre-merge point (593e45f)

[snoopycrimecop]

Conflicting PR. Removed from build DOCKER-merge#957. See the console output for more details.
Possible conflicts:

• PR docker-compose with merge-ci build #33 joshmoore 'Py2'
  • 50-config.py
• PR Custom startup command #34 manics 'Custom startup command'
  • 99-run.sh

--conflicts

[joshmoore]

Merging but not re-releasing since there are have been no changes since the first tag (unlike ome/omero-server-docker#33)