support secret/secret_file in knife.rb
support secret/secret_file in knife.rb

Fixes KNIFE-398
srirajan/knife-rackspace,vhl/knife-rackspace,tdooner/knife-rackspace,martinb3/knife-rackspace,philoserf/knife-rackspace,rackspace/knife-rackspace,chef/knife-rackspace,chef/knife-rackspace,decklin/knife-rackspace