Allow setting sso-server login_username_label

roles/float-infra-sso-server/templates/server.yml.j2

@@ -78,3 +78,6 @@ site_name: {{ sso_site_title }}
 {% if sso_favicon is defined %}
 site_favicon: {{ sso_favicon }}
 {% endif %}
+{% if sso_login_username_label is defined %}
+login_username_label: {{ sso_login_username_label }}
+{% endif %}