Config values for raft election and heartbeat tick

raft election and heartbeat tick values are updatable through env variables in docker engine after a recent change. Also, the default values for election tick was recently changed. Following is the list of issues that have come out from these recent changes:

1. Updating the store: when the values for election and heartbeat tick are changed, they are not updated in the cluster spec in the store. This is especially true on upgrade. Note that the changed values are actually applied to the cluster but not updated in the store. So, the user is not able to observe the change directly. Note that this issue exists in the case of upgrades or changing values using env vars
2. Consistency: user may specify different values for election and heartbeat tick on different manager nodes, which can lead to instability.
3. Reporting: scope TBD

Start by tracing how Docker Engine environment values for Raft election and heartbeat ticks reach the cluster spec and store during upgrades and manager configuration changes. Determine how differing manager values should be handled and define the reporting scope; done means applied values are observable in the store and inconsistent configurations are addressed.