fix(ansible): allow multiple custom secrets without namespaces

[lc525]

Fix a bug where defining multiple custom secrets in an extra-vars file without explicit namespaces would add the default namespace only to the last one, and fail the playbook.

Special notes for your reviewer:

Previously, each iteration of the modified set_fact would overwrite the seldon_custom_secrets variable entirely rather than updating it (appending the default namespace to each secret defined in custom_secrets).