Fix EZP-21701: ez_render_field should be able to use current template as parameter

https://jira.ez.no/browse/EZP-21701

## Description
Using `ez_render_field()` it is possible to specify the template that will be used using the optional parameters: 

```jinja
{{ ez_render_field( 
       content, 
       'my_field_identifier',
       { 'template': 'AcmeTestBundle:fields:my_field_template.html.twig' }
   ) }}
```

This PR allows to specify the current template as being the source of the block.
This is useful when you want to sporadically override a field template.

```jinja
{% extends "MyBundle::pagelayout.html.twig" %}

{% block content %}
    {# Note that "tags" is a field using ezkeyword fieldType #}
    <div class="tags">{{ ez_render_field( content, "tags" , { "template": _self } ) }}</div>
{% endblock %}

{# Here begins the inline block for my ezkeyword field #}
{% block ezkeyword_field %}
    {% spaceless %}
        {% if field.value.values|length() > 0 %}
        <ul>
            {% for keyword in field.value.values %}
            <li>{{ keyword }}</li>
            {% endfor %}

        </ul>
        {% endif %}
    {% endspaceless %}
{% endblock %}
```

## Tests
Functional test enhanced.

## Limitation
Like for [Symfony forms](http://symfony.com/doc/current/book/forms.html#global-form-theming), this will only work in templates extending another one.