Port cells extension to v3 API Part 2

Ports the cells extensions and the corresponding unittests to the
v3 framework.

Merges in functionality of the os-cells-capacities extension to
be the default behavior. Does not yet address v3 TODO of returning
capacity information in the cell detail method

Partially implements blueprint nova-v3-api

Change-Id: I24df708b9d6504a355fca0eacd0ffaeb6586610d
Merge "Port cells extension to v3 API Part 2"