Tools: Allow for download URL overrides

[rsenden]

Some customers may not allow downloading tools like ScanCentral Client or FoDUploader from external sites; instead they may want to host their own internal copies of these files. The 'fcli tool' commands should allow for overriding the standard download location,

More research is needed to identify the best approach for implementing this. For example:

• Should we allow for overriding the download location only, or also allow overrides of other download attributes like digest?
• Should we provide 'fcli tool config' commands for configuring overrides, and/or should we provide options on the individual 'install' commands to override download URL/digest/...?

Possibly easiest approach would be to allow customers to provide alternative YAML files in the same format as our internal download descriptors; an 'fcli tool config ...' subcommand would then copy the provided file into the fcli home directory. Then, when running an install command, fcli would first look in the fcli home directory to find the download descriptors, and otherwise use the shipped download descriptors.

[rsenden]

Latest fcli release supports customizable tool definitions that allow for alternative tool download locations, so closing this issue.