Test for blank node_attributes["group"], not empty

If node_attributes["group"] is nil, this would raise.
add apply_role_post_chef_call

This new hook mirrors apply_role_pre_chef_call, but is run after all the
chef-client runs are complete.  This can be used for things inserting
links into node.crowbar["crowbar"]["links"], since the links should not
show up until the chef-client runs succeeded.
Merge pull request #862 from aspiers/apply_role_post_chef_call

add apply_role_post_chef_call
Merge pull request #855 from bkutil/node_controller_check_blank_group

Test for blank node_attributes["group"], not empty