* Update Go client libraries for etcd
* Added etcd server container to run etcd3 tests automatically.
* Removed etcd2 test case: it fails the backend tests but the failure is
unrelated to the uplift. The etcd2 backend implementation does not
remove empty nested nodes when removing leaf (see comments in #11980).
Merge both functions for creating mongodb containers into one.
Add retries to docker container cleanups.
Require $VAULT_ACC be set to enable AWS tests.
Ben Ash