Cells: Add some cells support to admin_actions extension

Add clearing of instance state in a way that works with cells.

Implements blueprint nova-compute-cells

Change-Id: I1cc1560bceb842c3bc82c6e9510eaef8a4931293
Remove unused import.

Direct usage of the db has been eliminated from nova.service, so remove
the import.

Part of bp no-db-compute.

Change-Id: I909daccaf5dc0f26ad72914f90c9e2134e8decaf
Merge "Cells: Add some cells support to admin_actions extension"
Merge "Remove unused import."