Delete node/disk support

Partial-Bug: #1344486
Partial-Bug: #1391287
Feature: Support to remove node/nodes or remove a disk/disks from
    existing cluster
Support: Added functionality to provide the above. Fab commands,
    fab remove_storage_node and fab remove_disk are added to support
    the feature.
Test: Verified both the options with a 4 node cluster.

Change-Id: Ia644416f679c421f29d5b001c536dbfc7fe0215c
Merge "Delete node/disk support"