CHEF-4591: Knife commands to manipulate env_run_list on nodes

Unit tests show that the following functions have been added: 

knife role env_run_list add ROLE ENV cookbook(s) (optional - :after)
knife role env_run_list clear ROLE ENV
knife role env_run_list remove ROLE ENV cookbook
knife role env_run_list replace ROLE ENV old_cookbook new_cookbook
knife role env_run_list set ROLE ENV cookbook(s)
Also as part of the RSpec 3 update we switched to using `let` blocks.  Can you update all your ivars to use the `let(:knife) { Chef::Knife::RoleEnvRunListAdd.new }` form?