mgr/dashboard: Fix pool update on edit

mgr/dashboard: Fix pool update on edit

Issue: Pool field was blank on editing namespace form and user needs to type out pool
Reason: `image` field is no longer supported in form yet edit function trying to fetch it and failing on that, hence no following updates for pool
Fix: Removed stale `image` field

Additional changes:
- included unit tests for edit to capture such errors
- enhanced unit tests to sue ActivatedRouteStub and `router.url`
- pre populating pool form on create with first rbd pool in the list

Fixes https://tracker.ceph.com/issues/70966

Regression by: https://tracker.ceph.com/issues/69900

Signed-off-by: Afreen Misbah <afreen@ibm.com>