Adds live migration support to cells API

Fixes bug 1153087

DocImpact: Cells now allows live migration within a cell,
but not between cells.

Conflicts:
	nova/tests/compute/test_compute_cells.py

Change-Id: Ic8777c376883eaf6ff9cb4b379e0bacbfe5635b7
Merge branch 'cells_grizzly' of github.com:NeCTAR-RC/nova into cells_grizzly