[#64582784] retrieving gateway_interfaces from edge-gateway

added get_gateway_interface_by_id on edge-gateway. It finds the provider
or internal network by given id.
rename method name
removed unnecessary nil check
more integration tests for vcloud_gateway_interface_by_id

Added tests for following schenarios:
- test for getting vcloud attrs for orgVdcNetwork
- return nil if gateway_interface with given id not found
unit tests for vcloud_gateway_interface_by_id
version bump to 0.0.5
Merge pull request #5 from alphagov/get_gateway_interface

[#64582784] retrieving gateway_interfaces from edge-gateway