Return EdgeGateway network interface details

We need to access the EdgeGateway connected networks in order
to make decisions upon, and subsequently provide these details back
for the update_configuraion action.

The per-interface hashes are simplified and rubyfied, in order to shield
the user from the API content itself (available via vcloud_attributes)
Version bump
Clarify internal variables
update CHANGELOG
Rework around EdgeGatewayInterface class

Rather than passing around complex hashes translating
the Fog/vCloud GatewayInterfaceType datatype, instead return
an object representing this.

At present just handles the things we need right now: network_id, network_href
network_name.

In future, this object can handle questions like 'is this IP address in your list
of IP ranges?'.
Defensive programming for EdgeGatewayInterface

This is in effect input validation, so I feel is a temporary improvement
until we pull ConfigurationValidator into Core.

It's worthwhile having something in place whilst we wait for that though.
Merge pull request #7 from alphagov/65619490_network_lookup

[#65619490] EdgeGateway interface lookup