Add a MGM configuration entry to override the URI returned by the .well-known Tape REST API

For more details check the EOS Jira issue: https://its.cern.ch/jira/browse/EOS-5683

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This is an improvement to the implementation of the Tape REST API.

The idea is to add a MGM configuration entry to override the URI returned by the .well-known of the Tape REST API call (instead of having the mgm just sending it's hostname by default...).

This would allow us - for example - to redirect the traffic to a proxy instead of tying it to a specific MGM.