Add spec test for ThirdPartyPeerId

Verify the value of `ThirdPartyPeerId`, as determined by vCloud Walker's
interpretation of the data returned by Fog.

This new test confirms the existing behaviour and gives us more
confidence to make changes without breaking that behaviour.
Re-factor spec tests to use Let, rather than Before

Using Let is more consistent with how we use false parameters in other
tests, and makes it a bit easier to read. Re-factoring a bit to make the
tests easier to read, and more logical.
Add support for local IpSec VPN tunnels

Support local IpSec VPN tunnels, such as an intra-vDC tunnel.

Local tunnels provide both an `:Id` and a `:Name` key, as determined
from this debugging session (please note, private link):
https://github.gds/gist/mattbostock/ba95460918bd7d3ada5e#file-vcloud-walker-pry-rb-L47