mgr/feedback: make tracker URL configurable and stop hitting live tracker in tests

mgr/feedback: make tracker URL configurable and stop hitting live tracker in tests

The feedback module hard-coded tracker.ceph.com as the Ceph issue
tracker hostname. This meant the dashboard test
test_issue_tracker_create_with_invalid_key actually performed an
HTTPS POST against the live production tracker on every CI run.
Whenever tracker.ceph.com returned an unexpected response (5xx,
gateway error, transient failure), the test would fail on
completely unrelated pull requests.

Add a runtime module option 'mgr/feedback/tracker_url' that
controls which host the CephTrackerClient talks to. The default
keeps the existing behaviour (tracker.ceph.com). The dashboard
test now sets this option to an unreachable hostname in setUpClass
so create_issue fails fast with a ConnectionError instead of
depending on tracker.ceph.com being reachable.

Combined with the prior commit's error-handling fix, the test now
deterministically verifies that any tracker failure is surfaced as
HTTP 400, without requiring network access.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>