Fix management-center label value for helm charts

[wildnez]

Currently the default value of mancenter metadata label is too large to cause deployment failure with error similar to the below:

create Pod rahul-march15v2-helm-dep-hazelcast-enterprise-mancenter-0 in StatefulSet rahul-march15v2-helm-dep-hazelcast-enterprise-mancenter failed error: Pod "rahul-march15v2-helm-dep-hazelcast-enterprise-mancenter-0" is invalid: metadata.labels: Invalid value: "rahul-march15v2-helm-dep-hazelcast-enterprise-mancenter-55b5945595": must be no more than 63 characters

The default value needs to be truncated and also documented to inform the user about number of characters allowed for the deployment.