Allow for providing a customized semaphore container

It can be undesirable to have a globally shared sempahore
container, especially since oslo.concurrency can now be shared
among many disjoint applications and libraries.

When a single container is used it is now possible to have those
disjoint applications/libraries collide on the same sempahore names.
This is not a good pattern to continue with, so in order to move away
from it allow a custom container to be provided (which defaults to the
existing global one) so that users of oslo.concurrency may provide there
own container if they so desire.

Change-Id: I9aab42e21ba0f52997de3e7c9b0fea51db5c7289