Make file_line default to ensure => present

Make file_line default to ensure => present

The examples in the file_line resource documentation state the following
resource should work:

    file_line { 'sudo_rule':
      path => '/etc/sudoers',
      line => '%sudo ALL=(ALL) ALL',
    }

Without this patch the example does not work because ensure is not set
to present.

This patch fixes the problem by setting the default value of ensure to
present.