Avoid printing contribution message on user interruption

[ggiamarchi]

When vagrant is interrupted, basically with CRTL^C in the terminal, it print the message

An unknow error happened in Vagrant OpenStack provider

To easily debug what happened, we recommend to set the environment
variable VAGRANT_OPENSTACK_LOG to debug

    $ export VAGRANT_OPENSTACK_LOG=debug

If doing this does not help fixing your issue, there may be a bug
in the provider. Please submit an issue on Github at
https://github.com/ggiamarchi/vagrant-openstack-provider
with the stracktrace and the logs.

We are looking for feedback, so feel free to ask questions or
describe features you would like to see in this provider.

In such a situation, it should not.