Convert gateway integration tests to use expect

To follow [best practice](http://betterspecs.org/#expect).

This conversion was done by Transpec 2.1.0 with the following command:

```
transpec spec/integration/edge_gateway/edge_gateway_spec.rb
```

* 3 conversions
    from: == expected
      to: eq(expected)

* 3 conversions
    from: obj.should
      to: expect(obj).to

See also: https://github.com/yujinakayama/transpec#supported-conversions
Use vCloud Tools Tester for edge gateway tests

Convert the edge gateway integration tests to use the [vCloud Tools
Tester](https://github.com/alphagov/vcloud-tools-tester) gem now that
it's available for use as of 9b02ef0e66e294f1f60ab4828da009bbd66664fb.

* * *

Also, bump the required version of vcloud-tools-tester to 0.0.6 so that
we can use the new test parameters set as of that version.
Rename gateway_interface to network_interface

The integration tests for EdgeGateway#vcloud_gateway_interface_by_id
referred to each network interface they defined as `gateway_interface`.

To remove any confusion between an organisation vDC network interface
and the uplink ('external-facing') interface, rename this variable to
the more generic `network_interface` in those tests.
Add integration tests for Core::EdgeGateway

Add integration tests for the Vcloud::Core::EdgeGateway class:

  - EdgeGateway#get_by_name
  - EdgeGateway#get_ids_by_name
  - EdgeGateway.vcloud_attributes
  - EdgeGateway.name
  - EdgeGateway.href
  - EdgeGateway.interfaces

The 'edge_gateway_id' parameter depends on version 0.6.0 of the vCloud
Tools Tester gem.