qa/tasks/ragweed: always set ragweed_repo

qa/tasks/ragweed: always set ragweed_repo

* extract get_ragweed_branch() out of download() task, for better
  readablity.
* use a loop for retry when the first clone fails
* drop the `raise ValueError()` clause as it never happens. we could use
  an assert() here, but i don't think it is necessary anyway.
* use sh() instead of run() for better readablity.
* always set ragweed_repo. before this change this variable is
  unbounded if `force-branch` is set.

Fixes: https://tracker.ceph.com/issues/46771
Signed-off-by: Kefu Chai <kchai@redhat.com>