Update IceGrid templates to make the OMERO.tables module configurable

[sbesson]

Add a new top-level TABLES_MODULE variable used by the Tables template to start the new server.
Functionally, this will need to be tested alongside a corresponding OMERO.py with the logic to replace @omero.tables.module@ by either the value of the omero.tables.module configuration or the default runTables value.

Together with ome/omero-py#419, the following scenarios should be tested

• the default behavior should be unchanged i.e. starting the server either via omero admin start or the systemd service file should rewrite the IceGrid templates using the default runTables value and start the Tables service
• setting omero config set omero.tables.module runTables in the OMERO configuration and restarting the server should have an identical behavior
• setting omero config set omero.tables.module notexistingTables in the OMERO configuration and restarting the server should not start the Tables service and master.err should contain a line similar to /opt/omero/OMERO.venv/bin/python: No module named notexistingTables

[sbesson]

Probably the outstanding question is whether this mechanism is in scope for the upcoming OMERO.server 5.6.12 @jburel?

If so, an initial documentation step would be to add the property to etc/omero.properties in this repository and add a new Tables header similar to ome/omero-py#416 to register the new property in the configuration glossary of the reference OMERO documentation at https://omero.readthedocs.io/en/latest/sysadmins/config.html

[jburel]

I think it will make sense to include this strategy in the upcoming release