generator: pull dependency at last step of new

[hongchaodeng]

To simplify the workflow, we would also pull down the dependencies in new generator.

But there are two options to set up dep:

1. Render Gopkg.toml beforehand, since we know all the dependencies and can better control versions.
   Then do dep ensure.
2. Do dep init. This would rely on the tool to pick the versions and introduce unpredictability. It might be simpler and more generic if we don't know some dependencies beforehand.
   Then do dep ensure.

[fanminshi]

lgtm.

[hongchaodeng]

@fanminshi
I have updated the comments. After evaluation, we would go with option 1 initially. Do you think that would be good?

[fanminshi]

@hongchaodeng option 1 seems good with me. I like the fact that we are in control of certain dependencies so that we can ensure that operator-sdk will work properly.