Sort the groups on aws_iam_user

The IAM API returns the groups on a given user sorted lexically.  This
means that if you define a user like this:

    aws_iam_user { 'foo':
      ...
      groups => [ 'b', 'a' ],
      ...
    }

then Puppet will log spurious "changes" like so:

    notice: /Stage[main]/Aws_iam_user[foo]/groups: groups changed ['b', 'a'] to 'a b'

This can be easily solved by sorting the group names inside
`new_from_aws`.