The threshold should not be able to be set to a negative number.

Created by: wenxin-01

Describe the bug

Preset conditions 1、There is account A, which has cluster management privileges. 2、Use account A to log in to KS, and go to Platform management--》Cluster "B" management--》Alerting Policies.

To Reproduce Steps to reproduce the behavior:

1. Click ‘create’
2. Click 'next' after enter name
3. Select the alarm rule as ‘cpu average load’ 、'iops' 、‘Available memory’ or ‘throughput’
4. Enter a negative number in the threshold inputbox

Expected behavior Negative number cannot be entered

Actual behavior Negative numbers can be entered and can not be cleared

/priority medium /area alerting /cc @leoendless /kind bug /milestone 3.1.0