Swift formpost cleanup

Formpost test failed sometimes with an Unauthorized error.
A possible reason for this is the eventual consistency, because
the required account metadata might not be ready when the test
is executed.

Metadata availability is now checked in setUp(). This check is
already used in tempest.api.object_storage.test_tempurl.

Tests now also use the built in client. An additional negative test
has been added to ensure that formpost middleware is actually used
and not bypassed due to an already authenticated request.

Change-Id: I9fd2f3eeae4648015976d3c5944655b211ccbaa1
Closes-Bug: 1304341
Merge "Swift formpost cleanup"
Merge branch 'master' of https://github.com/openstack/tempest