Update Result.String implementation to work as declared in interface

[jellonek]

String() of Result declared in pkg/types/types.go should return a Result
representation in JSON, instead of custom formatted string, which
additionally is not in the same form as described in docstring, which also e.g. is missing Routes part.

This PR changes this behavior to be consistent with behavior documented in the interface.

As String() interface does not have any place for errors - if there
would be any - instead of JSON value output will be an empty string.

Closes #581

edit/update:
As discussed in comments - this PR removes funcs which are turning Result to string.

[rosenhouse]

Can we just delete this method? How are people using this method today?

[jellonek]

It's probably used only for debugging purpose, but removal will be breaking change. At least it would require a note in release notes.

[andrey-ko]

Probably types020.Result also should be updated for consistency:

cni/pkg/types/020/types.go

Line 87 in 8fa373b

func (r *Result) String() string {

[dcbw]

I vote for just removing the function entirely.

[jellonek]

+1
Ok. So I'll update PR with removal of this func and update in Spec.

[jellonek]

Bump. Funcs removed.
Please remember that it is a possibly breaking change, while we are not aware if anyone was using that.
Quick look on output of git grep "\\bString" in plugins repository doesn't show any uses of that in our code.

[bboreham]

LGTM
It's a breaking change; I guess will trigger 0.8.0 when we release.