custom jail config property namespace

[gronke]

Several users have reported interest in custom jail config parameters. libiocage refuses to store unknown jail config properties and warns when there are unknown ones defined in an existing configuration. To get around this issue configuring custom properties, libiocage should accept a namespace for such variables.

The namespace prefix used should be user., for example:

ioc set user.myproperty="Some Value" <JAIL>

[urosgruber]

@gronke that would be an awesome feature.