lookup_plugins: loop over hashes

Create a lookup plugin named dict that can be used to loop over hashes.
It converts a dict into a list of key-value pairs, with attributes named
"key" and "value."  Also adds a brief explanation and simple example to
the docs.

Signed-off-by: Kent R. Spillner <kspillner@acm.org>
Merge pull request #5964 from sl4mmy/with-dict

lookup_plugins: loop over hashes

Tests good with the global variables:
```
users:
  alice:
    name: Alice
    id: 12345
  luna:
    name: Luna
    id: 23456
```

and playbook tasks:
```
- name: Print records.
  debug: msg="User {{ item.key }} is {{ item.value.name }}  {{ item.value.id }}"
  with_dict: users
```