Make functional tests tolerate pre-existing versioned container

test/functional/tests.py:TestAccount.testAccountHead relies
on the account having a known number of containers. The test
setup attempts to delete all existing containers but this fails
a container has versions (and so is not emptied, and cannot be
deleted). The tests then fails because the expected number of
containers does not match the actual.

'bin/resetswift' before running tests will obviously clear all
state but is not always convenient.

This change removes any x-versions-location header before deleting
containers during test setUp.

Steps to recreate the pre-condition for failure on master:
(based on original work by clayg)

swift post target -r '.r:*, .rlistings'
swift post source -H 'x-versions-location: target'
for i in {1..4}; do
  echo "junk${i}" > junk
  swift upload source junk
done

Co-Authored-By: Clay Gerrard <clay.gerrard@gmail.com>
Change-Id: I3efb6c20dc1fb3e979087e8a93d04ba7e346b5b6

test/functional/swift_test_client.py

@@ -460,6 +460,7 @@ class Account(Base):
     def delete_containers(self):
         for c in listing_items(self.containers):
             cont = self.container(c)
+            cont.update_metadata(hdrs={'x-versions-location': ''})
             if not cont.delete_recursive():
                 return False