Add support for ENV of the form: ENV name=value ...

still supports the old form: ENV name value.

I have mixed feelings about this.  While it works and is kind of cool to be able to do:
ENV abc=def foo="feed the dog" hungry=eat\ a\ pizza late="go to bed 'now' I said"
and each key=value is parsed like you'd expect it to be on the command line, the ability to do more than
one ENV at a time really should be done under some kind of batching mechanism.  But, of course,
that's not there yet and we're not sure when it will be.  So, unlike the RUN command where we can
concat things together with &&'s, the ENV command doesn't have that option - not without this feature.

Closes #2333

Signed-off-by: Doug Davis <dug@us.ibm.com>
I've just tested (using `make docs`) and these indented code sections need an extra line feed:

```
For example:

    ENV myName="John Doe" myDog=Rex\ The\ Dog

and

    ENV myName John Doe
    ENV myDog Rex The Dog

will yield the same net results in the final container, but the first form
does it all in one layer.
```