Add ssl_key_file and ssl_cert_file attr to list of default Ubuntu attrib...

...utes

Running postgresql cookbook on Ubuntu 12.04 yields the following error when attempting to restart postgres during the initial chef run:

could not load server certificate file "server.crt": No such file or directory
   ...fail!

Full error output:
================================================================================
Error executing action `start` on resource 'service[postgresql]'
================================================================================


Mixlib::ShellOut::ShellCommandFailed
------------------------------------
Expected process to exit with [0], but received '1'
---- Begin output of /etc/init.d/postgresql start ----
STDOUT: * Starting PostgreSQL 9.2 database server
 * The PostgreSQL server failed to start. Please check the log output:
2014-03-12 21:26:53 GMT FATAL:  could not load server certificate file "server.crt": No such file or directory
   ...fail!
STDERR:
---- End output of /etc/init.d/postgresql start ----
Ran /etc/init.d/postgresql start returned 1

-----------------------------------------

Defining these attributes within the default_attributes hash was not populating the values within templates/default/postgresql.conf.erb as one would expect. 

default_attributes(
    'config' => {
      'ssl_key_file' => "/etc/ssl/private/ssl-cert-snakeoil.key",
      'ssl_cert_file' => "/etc/ssl/certs/ssl-cert-snakeoil.pem"
    }
  }
)

My commit adds two attributes to attributes/default.rb: ssl_key_file and ssl_cert_file which are the files for the default ssl package on Ubuntu: http://packages.ubuntu.com/saucy/ssl-cert