r/vmhost: Install mount helpers

Filesystems like NFS and CIFS require "helper" utilities (i.e.
`mount.nfs` and `mount.cifs`, respectively).  These need to be installed
in order for a system to be able to mount those filesystems.

The current shared storage system uses NFSv4, and as such, the
*nfs-utils* package needs to be installed on the VM hosts.

group_vars/vm-hosts.yml

@@ -12,6 +12,8 @@ libvirt_networks:
   type: bridge
   bridge_iface: mgmt
 storage_host: storage0.pyrocufflink.blue
+shared_volume_mount_packages:
+- nfs-utils
 mount_shared_volumes:
 - mountpoint: /var/lib/libvirt/images
   host: '[{{ lookup("dig", storage_host, "qtype=AAAA") }}]'