Add validation for duplicated mount point

Description
===========
If users create two cinder volumes with the same mount point on the container, the container failed to create. We can do better by validating this case and failed earlier for duplicated mount points.

Reproduce
=========
$ zun run --mount size=1,destination=/data --mount size=2,destination=/data nginx
....
$ zun show 8eaa089d-7c71-4f34-8d07-eb01e7fd4773
...
| status_reason | Docker internal error: 400 Client Error: Bad Request ("Duplicate mount point: /data"). ...

Expected
========
The first command (zun run ...) failed with 400 response

Actual
======
The first command succeeded. The container failed to create