Customise Windows service name: omero.windows.servicename

This is needed to allow multiple server deployments on the same Windows system.
```
C:\OMERO.server>bin\omero config set omero.windows.servicename toast

C:\OMERO.server>bin\omero config get
omero.data.dir=\OMERO
omero.windows.servicename=toast

C:\OMERO.server>bin\omero admin start
Found default value: c:\omero_dist\var\master
Attempting to correct...
Converting from c:\omero_dist to C:\OMERO.server
Changes made: 6
Creating C:\OMERO.server\var\master
Creating C:\OMERO.server\var\registry
No descriptor given. Using etc\grid\windefault.xml
Installing toast.master Windows service.
Successfully installed toast.master Windows service.
Starting toast.master Windows service.
Waiting on startup. Use CTRL-C to exit
..................
C:\OMERO.server>
```

This is currently deployed as a merge-deploy on Hake (OMERO-5.1-merge-deploy-win, port 4064, servicename: OMEROmerge) alongside latest-deploy (OMERO-5.1-latest-deploy-win, port 14064, default servicename since PR isn't merged). Note multiple web/IIS isn't working for other reasons so ignore web for now.

Test locally on your own VM if you're feeling brave.