[Apiserver decouple] Second part

[leonwanghui]

What this PR does / why we need it:
See design spec for detailed information.
Please notice that this PR would address Phase 2 of the design spec.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Release note:

[coveralls]

Coverage increased (+0.9%) to 38.081% when pulling 398148f on leonwanghui:second-part into 3a29829 on opensds:development.

[leonwanghui]

This patch contains quite big changes with below items:

• Merge controller and dock protobuf definition into model protobuf located in model module.
• Move all logic about status machine into controller module, except that some database operations when deleting volume group still remains in dock module.
• Change CreateAttachmentOpts and UpdateAttachmentOpts definition in protobuf to CreateVolumeAttachmentOpts and UpdateVolumeAttachmentOpts.
• Make some changes of volume group operations with dock driver interface, to make it more standard.
• Create pkg/model/proto/response.go file for Generating GenericResponse struct easily.
• Add some status operations in pkg/db/db.go.
• Simplify the ControllerGrpcServer construction by removing pkg/controller/server.go.
• Update e2e_test.go to make e2e test output more elegant.

[xing-yang]

@leonwanghui Can you address my comments, please? Also need a rebase.

[xing-yang]

@wisererik Please review this.

[wisererik]

LGTM

[wisererik]

@leonwanghui please rebase your PR, thanks

[xing-yang]

LGTM