rgw_sync_agent: fix a bunch of issues

rgw_sync_agent: fix a bunch of issues

- Read ceph.conf from stored copy that includes overrides
- Get system users and keys from cluster instead of reading other
  tasks' yaml, which may not be complete.
- Put zone info extraction from the cluster into utility functions,
  since it'll be useful for other tests later.
- Work with more than one agent on a single host
- Accept more than one client to run, like almost every other task
- Rename target to dest for consistency with radosgw-agent
- Don't make everything one large function

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>