[Symfony 3.4] Add support for "controller" keyword for configuring routes controllers

[Haehnchen]

# Current syntax
# It looks strange and exposes Symfony's internals
blog_show:
    path:     /blog/{slug}
    defaults: { _controller: AppBundle:Blog:show }

# Suggested syntax
blog_show:
    path:       /blog/{slug}
    controller: AppBundle:Blog:show

<?xml version="1.0" encoding="UTF-8" ?>
<routes xmlns="http://symfony.com/schema/routing" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

    <!-- Current syntax -->
    <route id="blog_list" path="/blog">
        <default key="_controller">AppBundle:Blog:list</default>
    </route>

    <!-- Suggested syntax -->
    <route id="blog_list" path="/blog" controller="AppBundle:Blog:list" />
</routes>

symfony/symfony#23227
symfony/symfony-docs#8240