Protect against a panic when we attempt to start a container with a name
that conflicts with an existing one.  If the existing one is being
deleted while nomad first attempts to create the container, the
createContainer will fail with `container already exists`, but we get
nil container reference from the `containerByName` lookup, and cause a
crash.

I'm not certain how we get into the state, except for being very
unlucky.  I suspect that this case may be the result of a concurrent
restart or the docker engine API not being fully consistent (e.g. an
earlier call purged the container, but docker didn't free up resources
yet to create a new container with the same name immediately yet).

If that's the case, then re-attempting creation will hopefully succeed,
or we'd at least fail enough times for the alloc to be rescheduled to
another node.

[ci skip]