probe AWS datacenter

probe AWS datacenter

S3: When initremote is given the name of a bucket that already exists,
automatically set datacenter to the right value, rather than needing it to
be explicitly set.

This needs aws-0.23. But, initremote stores the datacenter value, so
a remote set up this way can be used with git-annex built with an older aws.

This is not done when signature=anonymous, because in that case,
using AWS.defaultRegion works fine for accessing buckets on other
datacenters.

It feels a bit round-about to need to do this probing. But without it,
the problem seems to be that, with a v4 signature, the location constraint
is included in the Authorization header. When that is the wrong location,
AWS S3 rejects it. I do wonder though if there is an easier way that I
am currently missing.

Sponsored-by: Dartmouth College's DANDI project