Add back support for sigv4 overrides

This allows you to change the signature version used when
signing requests for a particular service.

This isn't strictly needed to communicate with various endpoints,
any region specific signature versions are captured in _endpoints.json.
However, with S3 we sometimes need to switch to sigv4 even if it
otherwise wouldn't be required.
Merge branch 'sigv4-switch' into develop

* sigv4-switch:
  Add back support for sigv4 overrides