[google] natIP is set to true when it must be an ip

As described in https://developers.google.com/compute/docs/reference/latest/instances networkInterfaces[].accessConfigs[].natIP
Fixes Fog::Errors::Error: Invalid value for field 'resource.networkInterfaces[0].accessConfigs[0].natIP': 'true'.  Must be an IP address
Merge pull request #2377 from maestrodev/google-natip

[google|compute] natIP is set to true when it must be an ip