For #313: Do not specify both `$APP` and `$IMAGE` for base commands.

As discussed in #313, dokku base commands such as build, release, and
deploy require specifying both `$APP` and `$IMAGE` as arguments where
`IMAGE="app/$APP"`. Thus, the `$IMAGE` parameter is currently redudant.

This patch removes the need to specify `$IMAGE` for these base commands
and is backwards compatible. If, in the future, the base commands expect
and `$IMAGE` different than `app/$APP`, then the `$IMAGE` parameter will
be re-added for those specific commands.
Merge pull request #321 from mikexstudios/remove-image-var

For #313: Do not specify both `$APP` and `$IMAGE` for base commands.