GCE: Set network tier, to avoid spurious changes
GCE: Set network tier, to avoid spurious changes

Otherwise we were seeing instance templates changing every time.

PREMIUM is the default, so we set it to PREMIUM for compatability.  In
future we may want to expose this option.

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