Add per-server identity migration

This is a first part for moving identity migration from separate
flow to the main server migration flow. Special flow is created
for migration of identity resources and added to the main server
migration flow. Thus we will ensure that necessary tenants and
users data are in store while migrating a server.
Remove identity flow from resources migration handler

As identity now migrated per-server, we should remove unnesessary
identity migration flow from handler and avoid multiple taskflow
engine runs during migration process.
Remove identity flow from resources migration in CLI script

Script no longer requires identity to be migrated prior to
migration of resources, as identity now being migrated per-server.
Refactor factory that creates server migration flow

Create single factory with common logic. Separate logic of creation
of image and snapshot in subprocedure of that factory.
Remove restricted clouds from args.
Restrict cloud access in secgroup tasks
Restrict access to owner of image in image migration tasks
Restrict access when booting server from image or snapshot
Restrict into user in secgroups migration
Pass user to secgroup migration flow builder
Unify reprovision server task
Add missing return value in server disk migration
Do not change the initial state of tasks

The tasks must not change initial clients to prevent any side effects
during potential reverts or retries.
Add placeholder user to store for migrating image
Fix long strings
Involve restriction clients in tasks