sync-pull: Check the actual repo readiness

sync-pull: Check the actual repo readiness

This has been broken for.. possibly ever.

--head results in 405 Method Not Allowed

Just a GET results in 200 no matter what, whether the repo exists or not.

```
++ curl -fsSo - --connect-timeout 5 --max-time 10 https://shaman.ceph.com/api/repos/ceph/reef/efac5a54607c13fa50d4822e50242b86e6e446df/rocky/10/flavors/default/
++ jq -r '.[].status // empty'
+ status=
+ '[' '' = ready ']'
+ echo 'Shaman repo for rocky/10 is not ready (status: unknown). Skipping.'
Shaman repo for rocky/10 is not ready (status: unknown). Skipping.
```

Signed-off-by: David Galloway <david.galloway@ibm.com>