templates: force OS selection in Boot OS form

Prevent backtrace when not OS is selected before the form submit.
Add 'required' attribute to the <input> elements of each OS option
in the Boot OS form.

ogcp/templates/actions/session.html

@@ -36,7 +36,7 @@ The selected clients have different installed OS:
                 {% for os_choice in form.os %}
                     <tr data-toggle="fieldset-entry">
                         <td class="radio-container">
-                            {{ os_choice(class_="form-control") }}
+                            {{ os_choice(class_="form-control", required=True) }}
                             <b>{{ os_choice.label.text }}</b>
                         </td>
                         <td>