Copy knife[x] to solo.rb: [:knife][x] 

Use case:
I like to keep private configurations like password out of node/*.json-files. 

In knife.rb:
```ruby
knife[:github_user_pass]  = {:user =>  "username", :password =>
"the_password"}
```
and then from any recipe:
```ruby
u = Chef::Config[:knife][:github_user_pass][:user]
pw = Chef::Config[:knife][:github_user_pass][:password] 
```