Switch to configmap leader election on 3.7 upgrade

This change sets the controllerConfig.election.lockName to
openshift-master-controllers on a 3.7 upgrade.

This is the default in a new 3.7 cluster.  Important excerpt from
the docs inside the origin codebase (slightly modified):

There are two modes for lease operation - a legacy mode that
directly connects to etcd, and the preferred mode which coordinates
on a configmap or endpoint in the kube-system namespace. Because
legacy mode and the new mode do not coordinate on the same key, an
upgrade must stop all controllers before changing the configuration
and starting controllers with the new config.

Signed-off-by: Monis Khan <mkhan@redhat.com>

playbooks/common/openshift-cluster/upgrades/v3_7/master_config_upgrade.yml

@@ -14,3 +14,8 @@
     dest: "{{ openshift.common.config_base}}/master/master-config.yaml"
     yaml_key: 'kubernetesMasterConfig.admissionConfig'
     yaml_value:
+
+- modify_yaml:
+    dest: "{{ openshift.common.config_base}}/master/master-config.yaml"
+    yaml_key: 'controllerConfig.election.lockName'
+    yaml_value: 'openshift-master-controllers'