Add translation for pending messages

This state would be reached if you are starting a box and it hasn't finished being set up, and from another command window you query `vagrant status`. For example, previously I got:

    Current machine states:
    
    default          translation missing: en.vagrant_aws.states.short_pending (aws)
    
    translation missing: en.vagrant_aws.states.long_pending

And after this patch I get:

    Current machine states:
    
    default                  pending (aws)

    The EC2 instance is still being initialized. To destroy this machine,
    you can run `vagrant destroy`.

Hope this helps.