'knife node run_list add' should take a list of RunListItems

'knife node run_list add' expects a single entry which it adds to the run_list. You should be able to provide a list of items. Currently if you try to provide multiple items you hit CHEF-1398.

Further, 'knife bootstrap' and 'knife ec2 server create' have been known to take a run list in different formatting. We should standardize on a single mixin for these. The knife ec2 plugin will have to wait until we ship a version of core shef that includes the mixin before switching.