Remove usage of internal library function in basic generator

Internal function shouldn't be used from the outside. Adds unit tests
to verify that the change has no other side effects.

Change-Id: I72d34c253816cfc4c05008f504c9abe6d2182efd
Closes-Bug: bug 1293543
Verify the response attributes of 'list_aggregates'

This patch verifies the response attributes of list_aggregates.
The response body of v2 and v3 API has the same format:

"aggregates": [
    {
        "availability_zone": "nova",
        "created_at": "2012-11-16T06:22:23.361359",
        "deleted": false,
        "deleted_at": null,
        "hosts": [],
        "id": 1,
        "metadata": {
            "availability_zone": "nova"
        },
        "name": "name",
        "updated_at": null
    }
]

Partially implements blueprint nova-api-attribute-test

Change-Id: If4c09929f3e9d3fd743aad2b727a74834fdbf8fd
Merge "Remove usage of internal library function in basic generator"
Merge "Verify the response attributes of 'list_aggregates'"