OpenstorageMigrate proto should be inline with Openstorage-Sdk

[ram-infrac]

Is this a BUG REPORT or FEATURE REQUEST?:
Enhancement
What happened:
Current proto definition of OpenstorageMigrate service does now follow sdk standard
https://github.com/libopenstorage/openstorage/blob/master/api/api.proto#L1590

What you expected to happen:
We should have separate Enumerate and Inspect call for querying migration statuses
Current Status() call should be deprecated