Add options to nova::compute::libvirt to use remove_unused parameters

Being able to modify the parameters "remove_unused_base_images", "remove_unused_original_minimum_age_seconds", "remove_unused_kernels", "remove_unused_resized_minimum_age_seconds" if desired.

For more informations in https://github.com/openstack/nova/blob/master/etc/nova/nova.conf.sample :

# Should unused base images be removed? (boolean value)
#remove_unused_base_images=true

# Unused unresized base images younger than this will not be
# removed (integer value)
#remove_unused_original_minimum_age_seconds=86400

# Should unused kernel images be removed? This is only safe to
# enable if all compute nodes have been updated to support
# this option. This will be enabled by default in future.
# (boolean value)
#remove_unused_kernels=false

# Unused resized base images younger than this will not be
# removed (integer value)
#remove_unused_resized_minimum_age_seconds=3600