CmdAdd and CmdEnv now respect Dockerfile-set ENV variables

Fixes #1136

Notes:

This changes the type of environment variable options.
The following dockerfile has the output that follows it.

    #Dockerfile. Directory also has a directory named 'testdir' which contains a file named 'myfile'.
    FROM ubuntu:12.10
    ENV DESTPATH /foobar
    ENV ORIGPATH ./testdir
    ENV FOOPATH $ORIGPATH/dir${DESTPATH}/dir
    ADD $ORIGPATH $DESTPATH/$ORIGPATH
    RUN echo $FOOPATH

    $ ls / | grep foobar
    foobar
    $ cd /foobar && ls
    testdir
    $ cd testdir && ls
    myfile