Verify certificate API response attributes

Now most attributes of Nova v2/v3 APIs are not checked in Tempest,
and this patch adds some tests which check these attributes to block
the backward incompatibility change in the future.

This patch adds the checks of certificate API responses.
The response body of this API is the following:
{
    'certificate': {
        'data': 'Certificate:\n    Data:\n        Version:...',
        'private_key': '-----BEGIN RSA PRIVATE KEY-----\nM...'
    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I5e97cfb5be56b4fa8de5d7262375b5b5b6de1b2a
orchestration api refactor access to stack outputs

Refactor test_server_cfn_init access to stack outputs so it uses
a new wrapper which gets the stack and returns the requested
output.  This provides a useful function which can be used in
other tests which parse the stack output.  Since we no longer
need to get the stack in test_server_cfn_init, remove the gets
and duplicate checks for CREATE_COMPLETE.

Change-Id: If1d0990ac9467b6e9778e4444b2ec5fc0f1e44e7
orchestration api add basic volume resource test

Adds a basic test which provides coverage for the OS::Cinder::Volume
resource, we just create and delete a volume and check the stack
details match those obtained directly via the cinder API.

Change-Id: Ia096f99c0c5decfafc144fe5afb47b6c871a218d
Defines a Credentials class

Multiversion auth part4

Introduces a Credential class.
Base class and V2 credentials are defined. V3 in the next patch.
Using Credentials, it becomes transparent for client managers and
tests which auth version is being used.
Defines a get_default_credentials utils, which is solely responsible
for reading credentials from CONF.
Defines a get_credentials, which builds V2 credentials
based on configuration.

Credentials are not used in tests yet, only by AuthProvider.
AuthProvider supports dict credentials in this patch for
backwards compatibility. Credentials are verified via unit test.

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

Change-Id: I5f8ab7ef37c99d2c2425b8487490e3517cfce7ed
Merge "orchestration api refactor access to stack outputs"
Merge "orchestration api add basic volume resource test"
Merge "Verify certificate API response attributes"
Merge "Defines a Credentials class"
Merge branch 'master' of https://github.com/openstack/tempest