Refactor UserGenerator

1. In the current implementation, it is possible for the UserGenerator to
   create a tenant in _create_tenant_users() and then fail at user creation,
   without returning the created tenant data. As a result, created tenants don't
   get deleted during cleanup.

   Here we refactor the tenants / users creation code by splitting it to two
   different methods, both launched via the Broker pattern.

   We remove the run_concurrent() method that is no longer used.

2. To fix a security issue, we change the password used for temporary users from
   "password" to a randomly generated uuid.

3. We rename the "concurrent" config option in the user context to
   "resource_management_workers".

Change-Id: I0b7f6b5677bb564c044c93aa1a9aeafc3b9f53cc
Closes-Bug: #1347215
Closes-Bug: #1365758
Merge "Refactor UserGenerator"