Updated Config and website-relation

- Changed Config to preserve previous values that don't exist in
  the config for the current hook. This allows saved values to be
  preserved indefinitely across many hook executions.
- Update website-relation-joined to save the relation id so that
  relation-set can be called fron config-changed if the meteor
  port changes (so that upstream reverse proxy can adjust accordingly)

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>