Fixed datasource config - allow empty password.

src/datasource-zabbix/partials/config.html

@@ -23,8 +23,7 @@
     <input class="gf-form-input max-width-21"
            type="password"
            ng-model='ctrl.current.jsonData.password'
-           placeholder="password"
-           required>
+           placeholder="password">
     </input>
   </div>