Backward compatibility to credentials in conf

Before switching to the YAML based configuration as the default for
credentials, we need to introduce backward compatibility to allow
credentials to be configured in tempest.conf.

Partially implements bp:test-accounts

Change-Id: I29c66c87d02f442b1d5b0b2d7c82ff2a140c111f
Migrate computev3 API tests to resource_* fixtures

Partially-implements bp:resource-cleanup

Change-Id: I581a9301d759c314e9f532caade580846d672287
Migrate identity API tests to resource_* fixtures

Partially-implements bp:resource-cleanup

Change-Id: I44e2661e5dfbb9a8267b912c704272861ebec17a
Migrate network API tests to resource_* fixtures

Partially-implements bp resource-cleanup

Change-Id: I455c372f7356343f4398f69255c6150c3521b953
Merge "Backward compatibility to credentials in conf"
Merge "Migrate computev3 API tests to resource_* fixtures"
Merge "Migrate identity API tests to resource_* fixtures"
Merge "Migrate network API tests to resource_* fixtures"
Merge branch 'master' of https://github.com/openstack/tempest