Before this fix, saving a :backend config gave the error
Can't save config "token" = "XXX" for on the fly backend ":backend"
Even when using the in-memory config `--config ""`
This fixes the problem by
- always using the in memory config if it is configured
- moving the check for a :backend config save to the file config backend
It also removes the contents of the config items being saved from the
log which saves confidential tokens being logged.
Fixes#5451