Break down methods in plan and instance controller

[takirala]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
Breakdown the instance controller and plan controller in to smaller parts to make the codebase more readable/reusable. Here is what I have done:

• Breakdown methods in plan and instance controller.
• Organise members in controller files by methods (most of these are for implementing an interface) and then followed by function (only some of them are pure functions currently)
• I initially planned on moving these functions to engine package but to decrease scope (and to avoid dealing with merge conflicts) i am pushing this for review after moving only a sample method to engine package.

Which issue(s) this PR fixes:

Partially addresses #229

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[fabianbaier]

closing in favor of #300

CC https://kubernetes.slack.com/archives/CG3HTFCMV/p1559754288056400?thread_ts=1559753481.046200&cid=CG3HTFCMV

[fabianbaier]

mea culpa! too many tabs

[kensipe]

@takirala where does this stand?

[kensipe]

Replaced by #563