Docker integration with libnetwork

- Updated Dockerfile to satisfy libnetwork GOPATH requirements.
    - Reworked daemon to allocate network resources using libnetwork.
    - Reworked remove link code to also update network resources in libnetwork.
    - Adjusted the exec driver command population to reflect libnetwork design.
    - Adjusted the exec driver create command steps.
    - Updated a few test cases to reflect the change in design.
    - Removed the dns setup code from docker as resolv.conf is entirely managed
      in libnetwork.
    - Integrated with lxc exec driver.

Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>