This commit contains fixes related to run Dask on lxplus
- in get_proxy, a file with the same name of the proxy BUT NOT VALID was being created; this happened more or less silently, since the object was there anyways (the failing cp message was tricky to spot)
- description of voms option was updated; X509_CERT_DIR environment variable needs to be set when running with --voms
- fix for dask coming from (https://mattermost.web.cern.ch/lpcrun2discuss/pl/i5gs94yu6j8wx8uttdbda9dwzo