Add a method to Installer classes to validate argument input
    
    There was no way to validate argument input from plugins and
    cause the installer to bail out. If a plugin needs to validate
    some input it can use the validate_args() method and raise
    ConfigurationError() if an issue is found.
    
    https://fedorahosted.org/ipsilon/ticket/78
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>