Add cross-cell resize policy rule and enable in API

This adds the "compute:servers:resize:cross_cell" policy
rule which is now used in the API to determine if a resize
or cold migrate operation can be performed across cells.

The check in the API is based on:

- The policy check passing for the request.
- The minimum nova-compute service version being high
  enough across all cells to perform a cross-cell resize.

If either of those conditions fail a traditional same-cell
resize will be performed.

A docs stub is added here and will be fleshed out in an
upcoming patch.

Implements blueprint cross-cell-resize

Change-Id: Ie8a0f79a3b16e02b7a34a1b81f547013a3d88996

doc/source/admin/configuration/cross-cell-resize.rst

@@ -0,0 +1,26 @@
+=================
+Cross-cell resize
+=================
+
+Train spec: https://specs.openstack.org/openstack/nova-specs/specs/train/approved/cross-cell-resize.html
+
+.. todo:: Flesh this out to describe what cross-cell resize is, how it is
+  triggered (policy), related configuration (long_rpc_timeout) including
+  the CrossCellWeigher, limitations and known issues, recovering from failures
+  during a cross-cell resize, maybe a flow chart for the overall process in
+  the code, minimum upgrade requirements and supported drivers (libvirt-only
+  at this time).
+
+Limitations
+~~~~~~~~~~~
+
+These are known to not yet be supported in the code:
+
+* Instances with ports attached that have bandwidth-aware resource provider
+  allocations.
+* Reschedules
+
+These are likely not to work since they have not been validated by testing:
+
+* Instances with PCI devices attached.
+* Instances with a NUMA topology.

doc/source/admin/configuration/index.rst

@@ -21,6 +21,7 @@ A list of config options based on different topics can be found below:
 
    /admin/configuration/api
    /admin/configuration/resize
+   /admin/configuration/cross-cell-resize
    /admin/configuration/fibre-channel
    /admin/configuration/iscsi-offload
    /admin/configuration/hypervisors

doc/source/admin/configuration/resize.rst

@@ -6,6 +6,8 @@ Resize (or Server resize) is the ability to change the flavor of a server, thus
 allowing it to upscale or downscale according to user needs.  For this feature
 to work properly, you might need to configure some underlying virt layers.
 
+For cross-cell resize, refer to :doc:`/admin/configuration/cross-cell-resize`.
+
 Virt drivers
 ------------