Added option to enable multiple HttpServices

Added option to enable multiple HttpServices, the configuration key is 'http_services' preserving the http_service confgiruation for backwards-compatibility.

In our use-case we have multiple http_servies to check with for every service have a different url and response +  code. This differs from https://github.com/liip/LiipMonitor/pull/8. This pull request creates multiple instances of the http_service service for every host.

A configuration now looks like:
<pre>
liip_monitor:
    checks:
        writable_directory: ['/tmp', '%kernel.logs_dir%']
        http_services:
          backend_api:
            host:                 %backend_api_host%
            port:                 80
            path:                 /my_api.php
            status_code:          200
            content:              im:ok
          a_service:
            host:                 %a_webservice_host%
            port:                 8080
            path:                 /
            status_code:          403
            content:              ~
</pre>