Dump 'tahoe backup' root dircap to stdout after completion if new --print-uri option present

[pataquets]

Corresponding ticket at https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2135

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This change is 

[remyroy]

Great code so far. I feel this would be quite useful for scripting backups.

There is still 2 failing backup related tests from this patch/pull request. The cli--tahoe-backup--dump-backup-dircap-stdout--tests branch from ​https://github.com/pataquets/tahoe-lafs/ seems to have some attempt at fixing those failing tests but they are still present.

There should also be an additional test in allmydata.test.test_cli.Backup to test this new command option.

Reassigning back the trac ticket to you for patch changes.

[tpltnt]

How should we proceed with this PR given the new plans for tahoe backup command?

[exarkun]

If these are the plans you mean, I think it's probably still fine to add this feature. It's a pretty simple change. This implementation has conflicts but I guess it's the work of about 5 minutes to resolve them (otoh there is no test coverage included). So I don't see a reason to keep it out. I'll review if someone else wants to get the implementation in shape.

[exarkun]

Over a year with no one taking me up on the review offer. I'm closing this for now. Anyone who feels like working on this should go for it. Re-open this PR or open a new one.