Add :data_bag_keyfile attribute to the LWRP.

Here is a rework of pull request #3 which allows an alternate keyfile to be specified. 
This time, the LWRP was extended to include a 'data_bag_keyfile' attribute which 
can specify a filename where the databag encryption key is stored. If the attribute is
not specified, the Chef default databag keyfile behavior is used (e.g. /etc/chef/encrypted_data_bag_secret).