Do not overwrite container volumes from config

Fixes #819 Use same persistent volume when a container is restarted
Testing, issue #1217: Add coverage testing into docker-ci
Merge pull request #1232 from dotcloud/1217-testing-coverage

Testing, issue #1217: Add coverage testing into docker-ci
Merge pull request #1207 from crosbymichael/819-use-persistent-volume

* Runtime: Do not overwrite container volumes from config
add support for container ID files (a la pidfile)
docs - add cidfile flag to run docs
docs - add example for cidfile
create the cidfile before creating the container

This change makes docker attempt to create the container ID file and
open it before attempting to create the container. This avoids leaving
a stale container behind if docker has failed to create and open the
container ID file.

The container ID is written to the file after the container is created.
Merge pull request #1159 from unclejack/add_container_id_file_to_run

*Client: Add support for container ID files (a la pidfile)