Fix 'puppet resource rabbitmq_binding' and add tests (#650)

[wyardley]

This should fix #650. It also

• Reformats the binding provider spec tests to be a little easier to read
• Adds a basic test for puppet resource rabbitmq_binding in the acceptance tests. This way, we should notice if a problem comes up with this command in the future.

Since we got feedback on how to test this in the type rather than in the create method in the provider, I can try to submit a separate PR to fix this for the rabbitmq_user resource.

Some of the other providers don't have instances implemented, so won't work with puppet resource; this is a separate issue, though refactoring them would be a good project for someone with the time on their hands 😉

[wyardley]

Spot checked some resources, and these look good:

rabbitmq_queue { 'queue1@host1':
  ensure => 'present',
}
rabbitmq_queue { 'queue2@host2':
  ensure => 'present',
}
[root@centos-7-x64 ~]# puppet resource rabbitmq_plugin
rabbitmq_plugin { 'rabbitmq_federation':
  ensure => 'present',
}
rabbitmq_plugin { 'rabbitmq_federation_management':
  ensure => 'present',
}
rabbitmq_plugin { 'rabbitmq_management':
  ensure => 'present',
}

let me know if there any that aren't working for extant resources.

[wyardley]

@alexjfisher @ekohl: I think this fixes it in a way that's closer to the original, thanks to @hunner's suggestion.

if !self[:source] && provider.source errored for me, but
if !self[:source] && !defined? provider.source seems to work.

I can switch the user provider to working that way again too when I get a chance.