Allow cellMods without custom systemd service

Hi guys,

Patch to allow cellMods without -l <path_to_cellMods> and consequently no need for a custom systemd service.

It allows us to use ce-deploy (i.e. new deploys will not overwrite our service).
Thus we can run tests in an environment closer to production - all the ansibles from ce-deploy will apply to us as well.

Note it no longer points to /opt/cellMods/. Instead, it looks for the ${E3_CMD_TOP}/cellMods/.
This should help us keep things organized (each of us can have its own copy without messing someone else's or overwriting same version numbers, etc...)

Thanks to @andreaspersson for the pathAdd EPICS_DRIVER_PATH trick.

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[edit] https://ce-deploy.esss.lu.se/iocs/3762