add facts showing available updates

Making use of the apt-check command from the 'update-notifier-common'
package (if available) display the number of available updates, number of
security updates as well as the update package names.
No need:

```ruby
irb(main):001:0> '1.7' > '2.1.0'
=> false
irb(main):002:0> '1.7.2' > '2.1.0'
=> false
irb(main):003:0> '1.7.2' > '2.1.'
=> false
irb(main):004:0> '1.7.2' > '2.1'
=> false
irb(main):005:0> '1.7.2' > '1.6'
=> true
irb(main):006:0> '1.7.2' > '1.6.0'
=> true
irb(main):007:0> '1.7.2' > '1.6.10'
=> true
```