pkg/generator: Add prune to Gopkg.toml template

[surajssd]

The code that gets vendored by dep by default is huge. We can
leverage the functionality given by dep viz. prune. This will remove
all the non-go code and unused packages.

It makes an exception to the k8s.io/code-generator repository. Since
we use the scripts from this repository to generate code.

Ref: https://golang.github.io/dep/docs/Gopkg.toml.html#prune

[surajssd]

cc: @hasbro17 @AlexNPavel

[hasbro17]

@surajssd Thanks for PR.
There might be use cases where an operator relies on vendored non-go files like a script or something.
I don't think the SDK should enforce pruning those by default. Users can always prune those themselves if needed.
Pruning tests and unused pkgs by default seem okay.
WDYT?

[surajssd]

@hasbro17 non-go also includes doc files like READMEs, etc.

IMHO we can always add an exception for the repos that need to have non-go stuff like in this PR:

operator-sdk/pkg/generator/templates.go

Lines 289 to 292 in 0c2d7aa

	[[prune.project]]
	name = "k8s.io/code-generator"
	non-go = false
	unused-packages = false

[hasbro17]

@surajssd I guess even without pruning non-go files, users already have to use the required tag to vendor non-go files from unused projects/pkgs.
So adding an exception for each such project along with that seems okay.

LGTM

[fanminshi]

lgtm