Set non-empty default mount options for the backup_mount resource.

[jperville]

The backup_mount resource passes an array of node['backup']['mount_options'] to the underlying mount resource since #27. Since these mount options are empty by default and they are passed verbatim to the underlying mount resource, not specifying any mount options explicitly results in broken entries in /etc/fstab.

For example:

root@bib:~# grep nfs /etc/fstab
172.17.17.10://mnt/HD/HD_a2//backups/myserver /data/backups-sync nfs  0 2
172.17.17.10://mnt/HD/HD_a2//backups/myserver /data/backups-sync nfs  0 2
172.17.17.10://mnt/HD/HD_a2//backups/myserver /data/backups-sync nfs  0 2
...

Note that each chef-run adds a line to the /etc/fstab file.
When the server restarts, the backup directory will not be mounted until the next chef-run.

This PR adds default, safe options (default['backup']['mount_options'] = ['soft', 'nosuid']) which should work out of the box.