Fix GCE instance lookup during validation
Fix GCE instance lookup during validation

ExternalID was deprecated and replaced by ProviderID, but the value of
ProviderID on GCE seems to have switched from instance ID to instance
Name.  Adopt a fallback matching strategy.

kubernetes/kops,robinpercy/kops,robinpercy/kops,gambol99/kops,blakebarnett/kops,aledbf/kops,justinsb/kops,robinpercy/kops,blakebarnett/kops,blakebarnett/kops,kubernetes/kops,gambol99/kops,aledbf/kops,gambol99/kops,justinsb/kops,gambol99/kops,blakebarnett/kops,aledbf/kops,robinpercy/kops,aledbf/kops,kubernetes/kops,justinsb/kops