remove deprecated ExternalID

[mikedanese]

/sig node
/sig cloud

[mikedanese]

ExternalID aliased to InstanceID

• azure
• cloudstack
• vsphere

ovirt InstanceID is ExternalID with a leading '/'

photon InstanceID and ExternalID are copies of the same function

gce InstanceID is <project>/<zone>/<instance> and ExternalID is <instance>

aws InstanceID is /<zone>/<instance> and ExternalID is <instance>