Container/Image/Conflig cleanup

Here a first  PR to clean up Container, Image and Config structs.

Problems:
*Config was used to both represents container's configuration and cli parameters.
*Some fields in images were unused or subject to change in the future, so we removed them.
*Some fields were only used by the client, and not meant to be save on disk

Changes:
*Some fields move from Config to HostConfig (`cpushare`, `networkDisabled`, etc...)
*We removed `comment`, `architecture` and `container` from Image
*A new struct was created to represents command line arguments not meant to be save on disk (`Attach*`, the `cidfile`, etc...)
*All the `dns/resolv.conf` stuff it now done at `start` and not at `create`
*lxc config file is now generated with only one template (as it needs both `container` and `hostConfig`)

**Please tell me if you have any comment**