Properly template list of hosts in playbooks.

In playbooks, hosts can be a YAML list. We templated the list before
converting it to a semicolon-separated string, which actually templated its
repr. This converts to a string first. A basic unit test is included.

test/hosts_list.yml

@@ -0,0 +1,5 @@
+# Test that playbooks support YAML lists of hosts.
+---
+- hosts: [host1, host2, host3]
+  tasks:
+  - action: command true