When running in GCE platform, an instance has access to:
http://metadata.google.internal/computeMetadata/v1/instance/network-interfaces/0/ip
Which gives back the private IP address, for example `10.240.0.2`.
http://metadata.google.internal/computeMetadata/v1/instance/network-interfaces/0/externalIp
Gives back the public Ip address, for example `130.211.108.21`.
As we have for `ec2`, we can support new network host settings:
* `_gce:privateIp:X_`: The private IP address of the machine for a given network interface.
* `_gce:hostname_`: The hostname of the machine.
* `_gce_`: Same as `_gce:privateIp:0_` (recommended).
Closes#13605.
Closes#13590.
BTW resolveIfPossible now throws IOException so code is also updated for ec2 discovery and
some basic tests have been added.
289cd5dcf4