Missing parameters should not affect vagrant user parameters

[maoueh]

When a user tries to do vagrant user parameters but there is missing parameters in the configuration, the command will crash saying a parameter is missing.

We could improve this. In the first time, find a way to avoid seeing the exception. Or introduce a new command to see missing parameters.

In the second time, key that could not be found should be reported in the output of vagrant user parameters directly. Something like displaying (missing) besides a key that is used but missing.

This issue could be hard to solve because of the way configuration values are parsed by Vagrant. Will see ...

[ghost]

#33 is closely related to this.