remote modify [ssh]: Add doc about the parameter 'allow_agent'

[gcoter]

This parameter was created per iterative/dvc/issues/4186 (for iterative/dvc#4186)

[jorgeorpinel]

From iterative/dvc/issues/4186:

with an SSH remote, sometimes I get an error like No existing session... This is only on one specific machine...
To fix it, I have to give the parameter allow_agent=False to paramiko.

OK so is this config option is meant for that one specific problem? (If so we can add a section about it to https://dvc.org/doc/user-guide/troubleshooting) Or can it be generalized? Let me keep digging... ⌛

[jorgeorpinel]

https://linux.die.net/man/1/ssh-agent

ssh-agent is a program to hold private keys used for public key authentication
(OpenSSH)

http://docs.paramiko.org/en/stable/api/client.html?highlight=allow_agent#paramiko.client.SSHClient.connect

Authentication is attempted in the following order of priority:

• The pkey or key_filename...
• Any key we can find through an SSH agent
• Any (default-named) key... in ~/.ssh/
• Plain username/password auth, if... given

So basically, setting allow_agent=false is a workaround for misconfigured SSH agents running in a machine, right?

[jorgeorpinel]

If so, here's my suggestion:

Suggested change

	- `allow_agent` - whether to connect to the SSH agent (default is `true`).
	Please refer to
	[Paramiko's documentation](http://docs.paramiko.org/en/stable/api/client.html?highlight=allow_agent#paramiko.client.SSHClient.connect)
	for more information.
	- `allow_agent` - whether to use [SSH agents](https://www.ssh.com/ssh/agent)
	(`true` by default). Setting this to `false` is useful when `ssh-agent` is
	causing problems, such as a "No existing session" error:

Notice that I decided to remove the link because I don't think DVC's authentication order of priority always reflects that of SSHClient.connect — it depends on how the remote is configured. But no strong opinion.

• And I do think we should add a Troubleshooting section for this, if others agree (doesn't have to happen in this PR though).

[jorgeorpinel]

OK I'm going to commit this and ask @shcheklein to merge it, if he agrees. But if anyone has feedback on this please lmk or feel free to continue editing the text. Thanks

[jorgeorpinel]

Thanks again, @gcoter !