signature v2 usage is forced for commands without a bucket name specified. (Ex.: ls)

[fviard]

When there is a request without a bucket defined, like for a simple bucket listing "ls", "signature v2" usage was is forced.

The fix will be to remove this legacy limitation, and issue first a signature v4 request in all cases.
But, to be noted, this fix might have a side effect on some random s3 compatible servers:
The problem is that for signature v4, we need to know the user's region.
And so, for things to work "automagically" without an user setting it, any s3 compatible server will have to behave like aws s3, ie providing the "region to use" in the reported error or otherwise just ignore the region value.

See: #1017