Install netcat-openbsd for live migration

netcat-openbsd is required on Debian/Ubuntu in order for live migration
to function. If it is not installed, you get an error that looks like:

libvirt.libvirtError: operation failed: Failed to connect to remote libvirt URI qemu+ssh://nova@hv1/system?no_verify=1&keyfile=/var/lib/nova/.ssh/id_rsa: End of file while reading data:
sh: 1: nc: not found: Input/output error
2020-05-19 22:13:32.553 4511 ERROR nova.virt.libvirt.driver [-] [instance: b5a313c0-326c-4e3f-bec4-22926289999f] Migration operation has aborted

This was originally fixed in [1] and then accidentally reverted in a
package cleanup[2].

[1] https://review.opendev.org/#/c/666759/
[2] https://review.opendev.org/#/c/662585/

Closes-Bug: #1833644

Change-Id: Ia2747142e6253b286d36d8c71a263ef7ed01341d

vars/debian.yml

@@ -52,6 +52,7 @@ nova_compute_packages:
   - genisoimage
   - kpartx
   - multipath-tools
+  - netcat-openbsd
   - nfs-common
   - open-iscsi
   - python3-guestfs

vars/redhat.yml

@@ -51,6 +51,7 @@ nova_compute_packages:
   - dosfstools
   - genisoimage
   - kpartx
+  - nc
   - python-libguestfs
   - sysfsutils