Fix initialisation of username on Windows (rebased onto develop)


This is the same as gh-539 but rebased onto develop.

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This is an intermittent fix for https://trac.openmicroscopy.org.uk/ome/ticket/9967. It make ``bin\omero admin start`` pick up the ``omero.windows.user`` setting properly and allows for auto-setting the Log On As user of the OMERO.master service.

Testing should verify that he Log On as username is set properly on the OMERO.master service:
- set up a user account in Windows for testing (e.g. ``dummy_user``)
- download and install OMERO on Windows,
- set ``omero.windows.user`` to dummy_user and provide the password in ``omero.windows.pass``,
- verify, that when doing ``bin\omero admin start`` the message ``Granting SeServiceLogonRight to service user ... `` appears

Second test case is the same set up, but instead of using OMERO config values (``omero.windows.user`` and ``omero.windows.pass``), use the ``bin\omero admin start -u <user_name>`` approach.

After the server starts up, open up ``services.msc`` and verify that the Log On as user for OMERO.master has changed.
When no user has been provided (e.g. ``bin\omero admin start``), the service should be run under the ``Local System`` account.