Fix API tests

Fix API tests

Patch https://review.openstack.org/#/c/256164/
broke the Neutron API job by unsetting Tempest configuration
values that were unused by Tempest, but used by Neutron's fork
of Tempest. This patch copies credentials from the 'auth' section
to the 'identity' section.

The longer term plan is detailed here:
https://etherpad.openstack.org/p/neutron-tempest-defork
TL;DR: Removed overlap of tests between the Tempest and Neutron
repos, then use Tempest's plugin architecture for the tests that
will remain in the Neutron repo.

Change-Id: Ieb396cfd17f9cb66776ead11e7f760c1c26a0b08