Add option to get the list of all platforms from non-default URL/file

at the moment we get the list of all known platforms from:

• /cvmfs/lhcb.cern.ch/lib/var/cache/LbPlatformUtils/all_platforms.json
• https://lhcb-couchdb.cern.ch/nightlies-release/_design/names/_view/platforms?group=true

but for testing it's useful to be able to specify a custom location