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.
    The current functionality provides support for OSD remove and
    node remove from an existing Ceph cluster. The LVM disk/host remove
    support is yet to be added.
Test: Verified both the options with a 4 node cluster.

Change-Id: I1a2c58766bd1e2a6bbad715f14cffb8b3c7a6b21
Merge "Delete node/disk support"