mgr/cephadm: fix objects_by_names initialization + some improvements

mgr/cephadm: fix objects_by_names initialization + some improvements

This commit include the following changes:

1. Fix objects_by_names initialization as we was using a dict for all
the case including global scoped objects which is not correct. For
those cases an instance of an empty TLSObject must be used.
2. Add sanity checks to the load() method to avoid loading incorrect
and malformed entries.
3. Add some helper functions to avoid code repetition

Fixes: https://tracker.ceph.com/issues/73625
Signed-off-by: Redouane Kachach <rkachach@ibm.com>