global role should not be able to imply domain-specific role

Global roles should only be able to imply other global roles, it should not be able to imply domain-specific roles. Domain-specific role visibility should be limited to its owning domain only.

To reproduce:

1. create a domain-specific role "foo_domain_role" in domain "foo".
2. create a global role "foo_admin".
3. PUT /v3/roles/<foo_admin_role_id>/implies/<foo_domain_role_id>
4. list imply roles for "foo_admin" and you'll see the imply relationship

vagrant@vagrant-ubuntu-trusty-64:~$ curl -s -H 'X-Auth-Token: 748aa5d5c13c4df2b8d6fb2075ca4c39' http://10.0.2.15:5000/v3/roles/45038d5e628b44c1857f33e839b06c77/implies | python -mjson.tool
{
    "role_inference": {
        "implies": [
            {
                "id": "306b6d6f97084df983a6f2fa30cf1163",
                "links": {
                    "self": "http://10.0.2.15/identity/v3/roles/306b6d6f97084df983a6f2fa30cf1163"
                },
                "name": "foo_domain_role"
            },
            {
                "id": "c256b7047f514515b3138d9efb594b21",
                "links": {
                    "self": "http://10.0.2.15/identity/v3/roles/c256b7047f514515b3138d9efb594b21"
                },
                "name": "bar_admin"
            }
        ],
        "prior_role": {
            "id": "45038d5e628b44c1857f33e839b06c77",
            "links": {
                "self": "http://10.0.2.15/identity/v3/roles/45038d5e628b44c1857f33e839b06c77"
            },
            "name": "foo_admin"
        }
    }
}
vagrant@vagrant-ubuntu-trusty-64:~$ curl -s -H 'X-Auth-Token: 748aa5d5c13c4df2b8d6fb2075ca4c39' http://10.0.2.15:5000/v3/roles/45038d5e628b44c1857f33e839b06c77 | python -mjson.tool
{
    "role": {
        "domain_id": null,
        "id": "45038d5e628b44c1857f33e839b06c77",
        "links": {
            "self": "http://10.0.2.15/identity/v3/roles/45038d5e628b44c1857f33e839b06c77"
        },
        "name": "foo_admin"
    }
}
vagrant@vagrant-ubuntu-trusty-64:~$ curl -s -H 'X-Auth-Token: 748aa5d5c13c4df2b8d6fb2075ca4c39' http://10.0.2.15:5000/v3/roles/306b6d6f97084df983a6f2fa30cf1163 | python -mjson.tool
{
    "role": {
        "domain_id": "0ba1cc88be31429d98866d101d1ed0ba",
        "id": "306b6d6f97084df983a6f2fa30cf1163",
        "links": {
            "self": "http://10.0.2.15/identity/v3/roles/306b6d6f97084df983a6f2fa30cf1163"
        },
        "name": "foo_domain_role"
    }
}