Access credential fields as attributes

Some tests access credentials attributes using dictionary
syntax (e.g. with get method); changed to use the Credentials
attribute directly.

Partially implements: bp multi-keystone-api-version-tests

Change-Id: Ic5b68af18d9a2dda581bdc1500d064e0ba732ec4
Merge "Access credential fields as attributes"
Add a common admin class for Nova v2/v3 API tests

For improving the maintenance, this patch changes a common admin class
to share it between v2 and v3 tests. In addition, this patch applies
this class to test_availability_zone of v2 and v3 as a sample.

After applying this class to all Nova API tests, we will be able
to remove current BaseV2ComputeAdminTest and BaseV3ComputeAdminTest
classes.

Partially implements blueprint nova-api-test-inheritance

Change-Id: Iede054d45c18922102cdf17938fde0e33f1df16c
Verify the response attributes of 'aggregate-add-remove-host' API

This patch verifies the response attributes of 'aggregate-add-remove-host'.
The response attributes of v2 and v3 are the same:
{
    "aggregate": {
        "availability_zone": "nova",
        "created_at": "2012-12-04T12:04:24.399784",
        "deleted": false,
        "deleted_at": null,
        "hosts": [],
        "id": 1,
        "metadata": {
            "availability_zone": "nova"
        },
        "name": "name",
        "updated_at": null
    }
}
The status_code is different:
v2 -> 200, v3 -> 202

Partially implements blueprint nova-api-attribute-test

Change-Id: I6f6bdc458f15f8af812fee6926e5133df09f9fa1
Merge "Verify the response attributes of 'aggregate-add-remove-host' API"
Merge branch 'master' of https://github.com/openstack/tempest