Make the displayed SSH username configurable

This patch allows the SSH username displayed in templates to be
configured through the SSH_USERNAME environment variable.

capsulflask/console.py

@@ -108,6 +108,8 @@ def detail(id):
   if vm is None:
     return abort(404, f"{id} doesn't exist.")
 
+  vm['ssh_username'] = current_app.config['SSH_USERNAME']
+
   if vm['deleted']:
     return render_template("capsul-detail.html", vm=vm, delete=True, deleted=True)