rgw: use a struct instead of std::tuple in AWS's VersionAbstractor.

rgw: use a struct instead of std::tuple in AWS's VersionAbstractor.

The C++11 standard mandates the variadic constructor of std::tuple
to be explicit. As a result, we can't use the uniform initialization
syntax in return statements. Moreover, the tuples became too verbose
after the AWSv4 rework.

See: https://stackoverflow.com/a/14961849
Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>