Improved fail_reason for cinder-backup swift connection errors

Improved fail_reason for cinder-backup swift connection errors

Modified swift backup service to catch socket errors when talking to
swift and raise a specific SwiftConnectionFailed exception in these
cases. This allows us to provide a more readable error message
detailing the problem connecting to swift to the end user when they
view the backup. Also reduced the default number of swift retries
so devstack environments fail faster - production environments can
tune these in cinder.conf.

Fixes bug: 1132791

Change-Id: Ibca744ea5adcbd31d068ac3d858bde6a4a0c9844