Enable Istio 1.4 in pull-knative-serving-istio-1.4-* test grids and r…

…emove 1.2 (#1559)

* Enable Istio 1.4 in pull-knative-serving-istio-1.4-* test grids and remove 1.2

This is similar request with knative/serving#5656 and #1394.

This PR has following changes:

[x] Enable Istio 1.4 as it is already included by knative/serving#6097
[x] Remove Istio 1.2 as it will be EOL on December 13th/2019
[x] Use Istio 1.4 for pull-knative-serving-integration-tests by default

* Use Istio 1.3 instead of 1.4 for pull-knative-serving-integration-tests

* Fix ci/prow/config_knative.yaml

* Fix cron time mismatch