Treat a False mandatory variable as defined.

When a variable is set to False, the "mandatory" filter returns False even if the variable is set. This is not expected behaviour. This patch will check if the variable exists and return True even if the value of the variable is False.

host_vars/vbox1.example.com
```yaml
var_test: no
```

host_vars/vbox2.example.com
```yaml
var_test: yes
```

tasks.yml
```yaml
- debug: var=var_test

- name: Test var
  file:
    state: touch
    path: /root/var
  when: "{{ var_test | mandatory }}"
```

Output.
```
TASK: [test | debug var=var_test] *********************************************
ok: [vbox1.example.com] => {
    "item": "",
    "var_test": "False"
}
ok: [vbox2.example.com] => {
    "item": "",
    "var_test": "True"
}

TASK: [test | Test var] *******************************************************
fatal: [vbox1.example.com] => Mandatory variable not defined.
changed: [vbox2.example.com] => {"changed": true, "dest": "/root/var", "gid": 0, "group": "root", "item": "", "mode": "0644", "owner": "root", "size": 0, "state": "file", "uid": 0}
```