Add support for default_region

Changes to actually use the
'default_region' setting in the
S3TEST_CONF file.
Previously, the first connection set
was assumed to be the default.
This means that the parse order
of the S3TEST_CONF file was dictating
how the tests behaved, resulting in
failures in some cases.

Signed-off-by: Joe Buck <jbbuck@gmail.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>