Use python3-guestfs in Ubuntu

This commit makes to use python3-guestfs instead of python-guestfs in
Ubuntu because python-guestfs package is not provided in focal[1][2].
This causes errors in some gate job if `ENABLE_FILE_INJECTION` is true
like the following.

```
...
Package python-guestfs is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  python3-guestfs

E: Package 'python-guestfs' has no installation candidate
...
```
http://paste.openstack.org/show/801073/

[1] https://packages.ubuntu.com/search?lang=en&keywords=python3-guestfs
[2] https://packages.ubuntu.com/search?lang=en&suite=default&arch=any&searchon=names&keywords=python-guestfs

Change-Id: Iffe60aa0351b732d543927afa1f1e846ba2a89fd

lib/nova_plugins/hypervisor-libvirt

@@ -100,7 +100,7 @@ function install_nova_hypervisor {
 
     if [[ "$ENABLE_FILE_INJECTION" == "True" ]] ; then
         if is_ubuntu; then
-            install_package python-guestfs
+            install_package python3-guestfs
             # NOTE(andreaf) Ubuntu kernel can only be read by root, which breaks libguestfs:
             # https://bugs.launchpad.net/ubuntu/+source/linux/+bug/759725)
             INSTALLED_KERNELS="$(ls /boot/vmlinuz-*)"