test rmi -f returns success

[andrewhsu]

- What I did

Added e2e test to verify docker rmi -f returns unix code success always because -f.

- How I did it

At my desk.

- How to verify it

$ make -f docker.Makefile test-e2e
...
=== RUN TestForceRemoveImage
--- PASS: TestForceRemoveImage (0.16s)
PASS

- Description for the changelog

N/A

- A picture of a cute animal (not mandatory but encouraged)

🐛

[codecov-io]

Codecov Report

Merging #567 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #567 +/- ##
==========================================
+ Coverage 49.4% 49.41% +0.01% 
==========================================
Files 208 208 Lines 17159 17159 ==========================================
+ Hits 8477 8479 +2 + Misses 8249 8247 -2 
Partials 433 433

[dnephin]

We already have test coverage for that here in the unit test suite: https://github.com/docker/cli/blob/master/cli/command/image/remove_test.go#L71-L78

Following https://github.com/docker/cli/blob/master/TESTING.md#guidelines I don't think we need an e2e test for the force flag.

I unit test for a successful docker rmi would be good, but it would go in the e2e/image package.