Add support for SSH Host Key Checking

By default it seems that SSH host key checking has been disabled. This
patch makes it optional. If a variable named known_hosts is passed in,
the key checking will be enabled. The variable should contain the
complete contents of the known_hosts file, which must contain the public
key(s) of the host(s) in the inventory.

action.yml

@@ -22,6 +22,9 @@ inputs:
   vault_password:
     description: The password used for decrypting vaulted files
     required: false
+  known_hosts:
+    description: Contents of SSH known_hosts file
+    required: false
   options:
     description: Extra options that should be passed to ansible-playbook command
     required: false