roll statefulsets on configmap updates

[t3mi]

This adds annotations for pod spec in statefulset for automatic rollout on config map updates per helm tip.

This change is 

[CLAassistant]

All committers have signed the CLA.

[darkn3rd]

@t3mi I tested this PR and found that the solution does not work, likely because the mount paths and file names are incorrect, i.e. /alpha/configs.yaml and /zero/configs.yaml.

If you would like to update this, the following items are needed:

• the mount path is /dgraph/config
• the file name can be json or yaml, depending what the user has for the key of alpha.configFile[0] and zero.configFile[0].
• there needs to be a boolean, e.g. alpha.configChecksum: true that enables or disables this feature.

I ran the following tests to test this feature:

git clone git@github.com:dgraph-io/charts.git && pushd charts && gh pr checkout 85 && popd
helm install t1 ./charts/charts/dgraph
helm upgrade t1 \
 --values ./charts/charts/dgraph/example_values/alpha-acl-secrets.yaml \
 --values  ./charts/charts/dgraph/example_values/alpha-acl-config.yaml \
 ./charts/charts/dgraph