Add filter dict_kv (#1264)

- Add filter `dict_kv` which returns a single key-value pair dictionary created from two arguments

Example 1

```
- hosts: localhost
  gather_facts: false
  vars:
    myvar: myvalue
  tasks:
  - debug:
      msg: "{{ myvar | dict_kv('thatsmyvar') }}"

OUTPUT:
ok: [localhost] => {
    "msg": {
        "thatsmyvar": "myvalue"
    }
}
```

Example 2

```
- hosts: localhost
  gather_facts: false
  vars:
    common_config:
      type: host
      database: all
    myservers:
    - server1
    - server2
  tasks:
  - debug:
      msg: "{{ myservers | map('dict_kv', 'server') | map('combine', common_config) }}"

OUTPUT:
ok: [localhost] => {
    "msg": [
        {
            "database": "all",
            "server": "server1",
            "type": "host"
        },
        {
            "database": "all",
            "server": "server2",
            "type": "host"
        }
    ]
}
```

plugins/filter/dict_kv.py

@@ -0,0 +1,70 @@
+# Copyright (C) 2020 Stanislav German-Evtushenko (@giner) <ginermail@gmail.com>
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+
+def dict_kv(value, key):
+    '''Return a dictionary with a single key-value pair
+
+    Example:
+
+        - hosts: localhost
+          gather_facts: false
+          vars:
+            myvar: myvalue
+          tasks:
+          - debug:
+              msg: "{{ myvar | dict_kv('thatsmyvar') }}"
+
+        produces:
+
+        ok: [localhost] => {
+            "msg": {
+                "thatsmyvar": "myvalue"
+            }
+        }
+
+    Example 2:
+
+        - hosts: localhost
+          gather_facts: false
+          vars:
+            common_config:
+              type: host
+              database: all
+            myservers:
+            - server1
+            - server2
+          tasks:
+          - debug:
+              msg: "{{ myservers | map('dict_kv', 'server') | map('combine', common_config) }}"
+
+        produces:
+
+        ok: [localhost] => {
+            "msg": [
+                {
+                    "database": "all",
+                    "server": "server1",
+                    "type": "host"
+                },
+                {
+                    "database": "all",
+                    "server": "server2",
+                    "type": "host"
+                }
+            ]
+        }
+    '''
+    return {key: value}
+
+
+class FilterModule(object):
+    ''' Query filter '''
+
+    def filters(self):
+        return {
+            'dict_kv': dict_kv
+        }