Addresses BP nova-independent-virt
Add the concept of a pluggable virt inspector that allows
polling of the hypervisor layer without relying on any
nova internals.
Note that the test-requires dependency on the nova master
tarball remains in place for the moment, as it's required
by the nova notifier tests.
Also we must leave the dependency on the libvirt-python
RPM or python-libvirt Debian package to the distros which
end up packaging ceilometer.
Change-Id: I20700320dd7e3196507173c780ab598b479e4021
40a3874c38