Add base class for all credential providers

Defines the base interface of all credential providers. User of this
will be the existing isolated_cread provider and the a new user_list
provider.

Change-Id: If382b549428c15ebb2ed850be9e0405e84226f6c
Partially-implements: bp/test-accounts
Migrate test_dashboard_basic_ops to tempest clients

Just switched from OfficialClientTest to ScenarioTest because
TestDashboardBasicOps does not use methods of OfficialClientTest.

Change-Id: I3b8eb90e058cde03cb40f120bb2c268f31ae078c
Partially-implements: bp:tempest-client-scenarios
Merge "Add base class for all credential providers"
Merge "Migrate test_dashboard_basic_ops to tempest clients"
Merge branch 'master' of https://github.com/openstack/tempest