[#70499746] 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

Also, simplify the existing test for EdgeGateway#update_configuration so that it tests the minimal required for vcloud-core without applying a complicated configuration.

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This supersedes alphagov/vcloud-core#66. Raising this from a personal fork to test the changes to our Travis configuration in alphagov/vcloud-core#69.