ioc allows to set unknown properties on `defaults`

[igalic]

Since #524 we've restricted unknown properties to its own user. namespace.
However! defaults allows us to cheat that, and we should fix that:

root@container-host1 /u/l/s/libiocage# ioc set unknonw_property=haha web-bf
The config property 'unknonw_property' of jail 'web-bf' is unknown.
root@container-host1 /u/l/s/libiocage# ioc set unknonw_property=haha defaults
Defaults updated: unknonw_property
root@container-host1 /u/l/s/libiocage# ioc set unknonw_property=hoho web-bf
Jail 'web-bf' updated: unknonw_property
root@container-host1 /u/l/s/libiocage#