Refactor Swift backup driver and introduce chunking driver

Refactor Swift backup driver and introduce chunking driver

Abstract out a "chunked" backup driver class between the top
abstract backup driver class and the Swift backup driver class
which implements common functionality for backup drivers that
store volume data in multiple "chunks" in a backup repository
when the size of the backed up cinder volume exceeds the size
of a backup repository "chunk."

The Swift driver itself is reworked to extend the chunked
backup driver and implement abstract methods defined in the
chunked backup driver.  It is expected that posix filesystem
and NFS backup drivers will also extend the chunked backup
driver.

Co-authors: Kevin Fox <kevin@efox.cc>
            Tom Barron <tpb@dyncloud.net>

Implements: blueprint chunked-backup-base-class

Change-Id: Ifbd1462ceb9b98bd8a6d72e5725399474eedf60e