Implement capability to extend existing volume.

This patch adds the core components to implement extending
the size of an existing available volume.

Volume status must be available, and the format is:
    extend <vol-id> <new-size>

where new-size must be > current size.

Adding support to drivers will be handled in follow up
patches for each of the existing drivers.

Implements blueprint: volume-resize

Change-Id: I40026083e564ea2074757e11e13cd07cdae3e6cc
Merge "Implement capability to extend existing volume."