Avoid TypeError when loading libvirt.LibvirtDriver
Avoid TypeError when loading libvirt.LibvirtDriver

Fixes bug 1078304

The libvirt.LibvirtDriver.__init__() signature has changed since:

  https://github.com/openstack/nova/commit/081b652c

to take a new virtapi parameter. This is not required by any
ceilometer usage of the driver, however the absence of a
corresponding argument causes importutils.import_object_ns()
to fail with TypeError.

For testing the nova fake.FakeVirtAPI still uses the old
signature, hence we try both.

Change-Id: I05a9728ceb435a72f5fa8a648cd70b079126396e

idegtiarov/ceilometer,pkilambi/ceilometer,rackerlabs/instrumented-ceilometer,rackerlabs/instrumented-ceilometer,m1093782566/openstack_org_ceilometer,tanglei528/ceilometer,mathslinux/ceilometer,maestro-hybrid-cloud/ceilometer,JioCloud/ceilometer,eayunstack/ceilometer,tanglei528/ceilometer,redhat-openstack/ceilometer,rickerc/ceilometer_audit,cernops/ceilometer,fabian4/ceilometer,MisterPup/Ceilometer-Juno-Extension,fabian4/ceilometer,pkilambi/ceilometer,shootstar/ctest,isyippee/ceilometer,cernops/ceilometer,openstack/ceilometer,dreamhost/ceilometer,JioCloud/ceilometer,maestro-hybrid-cloud/ceilometer,ityaptin/ceilometer,m1093782566/openstack_org_ceilometer,eayunstack/ceilometer,mathslinux/ceilometer,MisterPup/Ceilometer-Juno-Extension,sileht/aodh,citrix-openstack-build/ceilometer,Juniper/ceilometer,openstack/aodh,luogangyi/Ceilometer-oVirt,redhat-openstack/ceilometer,Juniper/ceilometer,rickerc/ceilometer_audit,chungg/aodh,r-mibu/ceilometer,froyobin/ceilometer,ityaptin/ceilometer,tanglei528/ceilometer,chungg/aodh,isyippee/ceilometer,froyobin/ceilometer,shootstar/ctest,rackerlabs/instrumented-ceilometer,pczerkas/aodh,dreamhost/ceilometer,idegtiarov/ceilometer,citrix-openstack-build/ceilometer,r-mibu/ceilometer,openstack/ceilometer,luogangyi/Ceilometer-oVirt,rickerc/ceilometer_audit,sileht/aodh,NeCTAR-RC/ceilometer,citrix-openstack-build/ceilometer,NeCTAR-RC/ceilometer,JioCloud/ceilometer,openstack/aodh,NeCTAR-RC/ceilometer,pczerkas/aodh