radosgw-agent.py: refactor, enable overrides

Refactored the radosgw-agent.py code so that it
is structured more like existing teuthology
tasks.
Additionally, added code to enable:
using the override field in YAML files,
specifying which radosgw-agent github branch
to use checkout and for the YAML file to
specify one of the following: a full sync,
an incremental sync and the starting of the
test radosgw-agent server (previously the
server was always started by this task).

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