rgw/bucket_logging: fix dangling string_view from ternary temporary

rgw/bucket_logging: fix dangling string_view from ternary temporary

Clearing a compilation warning:

The ternary operator created a temporary std::string to unify
const std::string& and const char* branches, leaving the
string_view pointing to a destroyed object. Replace with
find() + if/else to assign directly from stable storage.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>