Better error message if a provider does not support :public_address capability

[mkuzmin]

OpenStack provider does not expose :public_address capability, so Vagrant fails with an error:

ERROR vagrant: Vagrant experienced an error! Details:
ERROR vagrant: #<Vagrant::Errors::CapabilityNotFound: The capability 'public_address' could not be found. This is an internal error
that users should never see. Please report a bug.>
ERROR vagrant: The capability 'public_address' could not be found. This is an internal error
that users should never see. Please report a bug.
ERROR vagrant: /opt/vagrant/embedded/gems/gems/vagrant-1.7.4/lib/vagrant/capability_host.rb:94:in `capability'

Maybe there is a way to handle such case better, and display more descriptive error message.