Support iteration over command output in with_items.

When the output of a command is stored in a register, this will create a
stdout_lines field in the result object that contains stdout split into a list
of lines.  This list can then be iterated over using with_items.

test/results_list.yml

@@ -0,0 +1,19 @@
+---
+# Test iterating over lines of stdout stored in a register.
+- hosts: localhost
+  vars:
+    small_file: /etc/resolv.conf
+    temp_file: /tmp/ansible_result_list.tmp
+
+  tasks:
+  - action: command cat $small_file
+    register: result
+
+  - action: file dest=$temp_file state=absent
+
+  - action: shell echo '$item' >> $temp_file
+    with_items: ${result.stdout_lines}
+
+  - action: command diff $small_file $temp_file
+
+  - action: file dest=$temp_file state=absent