integration-cli: generate unix-style volume paths

Some tests in `docker_api_containers_test.go` assume the
docker daemon is running at the same machine as the cli
and uses `ioutil.TempDir` to create temp dirs and use them
in the test.

On windows ioutil.TempDir and os.TempDir would create win-style
paths and pass them to daemon. Instead, I hardcoded `/tmp/` and
generate some random path manually and allow daemon to create
the directory.

Fixes tests:
- TestContainerApiStartVolumeBinds
- TestContainerApiStartDupVolumeBinds
- TestVolumesFromHasPriority

Downside:
- Does not clean the temp dirs generated on the remote daemon
  machine unless delete container deletes them.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>