Child's root provider is not updated

Description
===========

You can update a resource provider(old root RP)'s parent RP from None to a specific existing RP(original root RP).
But if the resource provider(old root RP) has a child RP, the child RP's root RP is not updated automatically to the new root RP.

Reproduction
============
1. There is already an RP

```
* original_root
```

{
    "resource_providers": [
        {
            "uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed",
            "parent_provider_uuid": null,
            "generation": 1,
            "root_provider_uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed",
            "name": "original_root"
        }
    ]
}

2. create a new RP and its child using POST /resource_providers

```
* original_root
* old_root_rp
     |
     +-- child_rp
```

{
    "resource_providers": [
        {
            "uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed",
            "parent_provider_uuid": null,
            "generation": 1,
            "root_provider_uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed",
            "name": "original_root"
        },
        {
            "uuid": "6985934e-0d44-404e-9b59-92d33f89d9ef",
            "parent_provider_uuid": null,
            "generation": 0,
            "root_provider_uuid": "6985934e-0d44-404e-9b59-92d33f89d9ef",
            "name": "old_root_rp"
        }
        {
            "uuid": "b80b63c9-1923-42ac-8659-e32479c70eaf",
            "parent_provider_uuid": "6985934e-0d44-404e-9b59-92d33f89d9ef",
            "generation": 0,
            "root_provider_uuid": "6985934e-0d44-404e-9b59-92d33f89d9ef",
            "name": "child_rp"
        }
    ]
}

3. Update old root rp's parent to the original root using PUT /resource_providers/6985934e-0d44-404e-9b59-92d33f89d9ef

```
* original_root
    |
    +-- old_root_rp
             |
             +-- child_rp
```

{
    "resource_providers": [
        {
            "uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed",
            "parent_provider_uuid": null,
            "generation": 1,
            "root_provider_uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed",
            "name": "original_root"
        },
        {
            "uuid": "6985934e-0d44-404e-9b59-92d33f89d9ef",
            "parent_provider_uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed", # updated :)
            "generation": 0,
            "root_provider_uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed", # updated :)
            "name": "old_root_rp"
        },
        {
            "uuid": "b80b63c9-1923-42ac-8659-e32479c70eaf",
            "parent_provider_uuid": "6985934e-0d44-404e-9b59-92d33f89d9ef", # not updated :(
            "generation": 0,
            "root_provider_uuid": "6985934e-0d44-404e-9b59-92d33f89d9ef", # not updated :(
            "name": "child_rp"
        }
    ]
}

The old_root_rp's root provider uuid is updated,
but the child_rp's root provider uuid remains old root rp's uuid.

Expected
========

The child_rp's root provider uuid is also updated to the original_root's rp uuid.

{
    "resource_providers": [
        {
            "uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed",
            "parent_provider_uuid": null,
            "generation": 1,
            "root_provider_uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed",
            "name": "original_root"
        },
        {
            "uuid": "6985934e-0d44-404e-9b59-92d33f89d9ef",
            "parent_provider_uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed",
            "generation": 0,
            "root_provider_uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed",
            "name": "old_root_rp"
        },
        {
            "uuid": "b80b63c9-1923-42ac-8659-e32479c70eaf",
            "parent_provider_uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed", # updated as well :)
            "generation": 0,
            "root_provider_uuid": "da9bd8c5-e376-4828-b8ed-080081f8e4ed", # updated as well :)
            "name": "child_rp"
        }
    ]
}