s3: add option to disable ssl verification

[efiop]

I.e.

s3 = boto3.client('s3', verify=False)

Might be useful for people with strict ssl policies at a workplace.

[ghost]

@efiop , could you describe a little bit more the use case?

I don't think it should be a good idea to disable verifying the certificates; there's another option use_ssl, tho, that will disable SSL.

For the ones who have custom endpoints without SSL certificates, for example: http://localhost:8000 it will not force SSL usage.

[ghost]

In case we implement this, I'm imagining something like: dvc remote modify bucket disable_ssl true, what do you think?

[efiop]

I was actually thinking about something like dvc remote modify mys3remote verify_ssl false(true by default`).

[efiop]

Fixed by #1428