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 "Defines a Credentials class"