Add CONFIG_SCHEMA to dummy provider

Also added test DummyProviderTestCase.test_invalid_config,
DummyProviderTestCase.test_invalid_credentials and SetUp
method, where config was defined.

Change-Id: Idae2461ecf66d176705573af373de325d43bb4ba
Related: blueprint common-config-validation
Merge "Add CONFIG_SCHEMA to dummy provider"