Cluster Setup doesn't set consistent admin user

Similar to #1752, if you use the cluster setup wizard to create admin users on new nodes in the cluster, the created admin user will not use an identical salt on every node, meaning Fauxton fails to obtain a valid `_session` unless you manually distribute the ini file `[admins]` user yourself using a config management tool.

## Expected Behavior
Setup wizard creates a real server admin user that is usable from Fauxton immediately.

## Current Behavior
Setup wizard creates an admin user that cannot be load balanced because the salt is different.

## Possible Solution
Pass the salt along as part of the cluster setup API endpoint?

/cc @janl

Start at the cluster setup API endpoint and trace how the setup wizard creates the server admin in the [admins] configuration. Check the resulting _session behavior across nodes and define completion as a wizard-created admin that Fauxton can use immediately when requests are load balanced.

Written by the indexing model from the issue text.