Allow Upstart jobs on Amazon Linux

Amazon Linux is a RedHat derivative and comes with support for Upstart built in.  The current Upstart requirements assume the RedHat versioning scheme, but Amazon Linux does not use the same versioning scheme.

This change allows the use of Upstart jobs on Amazon linux.
This confine takes a list of expressions that can determine if the given provider is suitable, so instead of doing this you could do

```ruby
confine :any => [
  Facter.value(:operatingsystem) == 'Ubuntu',
  (Facter.value(:osfamily) == 'RedHat' and (Facter.value(:operatingsystemrelease) =~ /^6\./),
  Facter.value(:operatingsystem) == 'Amazon')
]
```