Use new patch-style config to the service environment variables

As of openflexure-microscope-server!420 (merged) we can use patch style configurations. The ofm_config.json can be changed to

{
    "base_config_file": "/var/openflexure/application/openflexure-microscope-server/ofm_config_full.json"
}

NOTE: Previously we were going to set $OFM_LIB directly in the file and set the OFM_LIB environment variable in the service file. However this could be confusing as it implies that the users env variables can be used, and the service env variable and the user env variables could end up out of sync.