mimic: update s3-test download code for s3-test tasks

mimic: update s3-test download code for s3-test tasks

Fixes: https://tracker.ceph.com/issues/43077
- Ensure the download code for all tasks running
s3-tests is consistent.
- Simplify download code to only use the config
variable 'force-branch' for the branch being
cloned.
- Make ceph-mimic the force-branch for all
suites using s3-tests.
- Add force-branch to suites running s3readwrite
& s3roundtrip tasks

Signed-off-by: Ali Maredia <amaredia@redhat.com>