Add virt driver capabilities definition

Going forward with the early stages of no-db-compute, we will
consistently hit cases where we need to eject some database usage
from one virt driver up a few layers. Since those are not commonly
used by all, some way of determining (and the higher layer) the
properties of the virt driver being used will be useful.

We have discussed using something like a mixin interface class
and multiple inheritance. We have discussed tricks like detecting
if the manage_image_cache() method has been overridden. We have
discussed trying it once and catching NotImplementedError and then
never trying again. Aside from the first, none of these are
unsneaky enough to bite us later.

This approach will provide us a way to declare such properties
succinctly in the compute driver to help the higher layers know
what we want them to do on our behalf.

Change-Id: I74dea9322a5b4688319ebf5d9afe416e93401c58

nova/compute/manager.py

@@ -2970,6 +2970,8 @@ class ComputeManager(manager.SchedulerDependentManager):
     def _run_image_cache_manager_pass(self, context):
         """Run a single pass of the image cache manager."""
 
+        if not self.driver.capabilities["has_imagecache"]:
+            return
         if FLAGS.image_cache_manager_interval == 0:
             return
 

nova/virt/driver.py

@@ -88,6 +88,10 @@ class ComputeDriver(object):
 
     """
 
+    capabilities = {
+        "has_imagecache": False,
+        }
+
     def init_host(self, host):
         """Initialize anything that is necessary for the driver to function,
         including catching up with currently running VM's on the given host."""

nova/virt/libvirt/driver.py

@@ -258,6 +258,10 @@ def _get_eph_disk(ephemeral):
 
 class LibvirtDriver(driver.ComputeDriver):
 
+    capabilities = {
+        "has_imagecache": True,
+        }
+
     def __init__(self, read_only=False):
         super(LibvirtDriver, self).__init__()