Bump CoreOS stable to latest version

1298.6.0 fixes some sporadic network issues. It also includes docker
1.12.6 which includes several stability fixes for kubernetes.

Signed-off-by: Sergii Golovatiuk <sgolovatiuk@mirantis.com>

.gitlab-ci.yml

@@ -251,7 +251,7 @@ before_script:
 .coreos_calico_sep_variables: &coreos_calico_sep_variables
 # stage: deploy-gce-part1
   KUBE_NETWORK_PLUGIN: calico
-  CLOUD_IMAGE: coreos-stable-1235-6-0-v20170111
+  CLOUD_IMAGE: coreos-stable-1298-6-0-v20170315
   CLOUD_REGION: us-west1-b
   CLUSTER_MODE: separate
   BOOTSTRAP_OS: coreos
@@ -290,7 +290,7 @@ before_script:
 .coreos_canal_variables: &coreos_canal_variables
 # stage: deploy-gce-part2
   KUBE_NETWORK_PLUGIN: canal
-  CLOUD_IMAGE: coreos-stable-1235-6-0-v20170111
+  CLOUD_IMAGE: coreos-stable-1298-6-0-v20170315
   CLOUD_REGION: us-east1-b
   CLUSTER_MODE: default
   BOOTSTRAP_OS: coreos