Detect central/decentral ingestion more generally

[sbliven]

Currently the check for data availability (assuming neither --copy nor --nocopy are set) is PSI specific and has a couple problems:

1. The ssh <host> test -d <sourceFolder> requires valid login credentials. It fails without kerberos or --pass.
2. It doesn't prefix the sourceFolder properly for merlin6 or merlin7
3. It only works from within the PSI network

Edit: prefixing is configured from pb-archive, so this works.