__init__: explicitly set default connection

The default connection was being set
implicitly (in whatever order the file
S3TEST_CONF was parsed in). It
is now set explicitly. The s3tests.py
teuthology task sets the 'default_region'
in the 'fixtures' section of S3TEST_CONF.
This patch uses that to set it in the
objects that drive the s3 tests.

Signed-off-by: Joe Buck <jbbuck@gmail.com>
testing a change

Signed-off-by: Joe Buck <jbbuck@gmail.com>