Fixing two issues in jenkins::config

1) For the Debian family (Checked Debian Wheezy and Ubuntu Precise) the defaults folder is in /etc/default instead of /etc/defaults
2) Removed prefixing of JENKINS_ on each variable as it removes flexibilty. If a user wants to add that - they can. This now enables me to set things like
HTTP_PORT correctly. Also - as a word of caution, file_line adds new resources to the end of the file - This may cause issues (For example, if HTTP_PORT is set after
JENKINS_ARGS then HTTP_PORT will not actually be read in, as JENKINS_ARGS calls it.

Tested on Ubuntu Precise 12.04 (Only checked the default part on Wheezy, didn't test - Can't imagine it being any different though)

  class { 'jenkins':
    config_hash => {
      'HTTP_PORT' => { 'value' => 2222 }
    }
  }

Was applied to my test node successfully.