chore: update the format of random password

[ShiChangkuo]

What this PR does / why we need it:

Which issue this PR fixes: (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

RandomPassword will generate a 12-character random password. The password format is uppercase letters*3 + lowercase letters*3 + special characters*2 + numbers*4 You can use "customChars" to overwrite the special characters.

Special notes for your reviewer:

Release note:

PR Checklist

• [x] Tests added/passed.
• [ ] Documentation updated.
• [ ] Schema updated.

Acceptance Steps Performed

$ make testacc TEST="./huaweicloud/services/acceptance/dws" TESTARGS="-run TestAccDwsExtDataSource_basic"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/dws -v -run TestAccDwsExtDataSource_basic -timeout 360m -parallel 4
=== RUN   TestAccDwsExtDataSource_basic
=== PAUSE TestAccDwsExtDataSource_basic
=== CONT  TestAccDwsExtDataSource_basic
--- PASS: TestAccDwsExtDataSource_basic (1458.42s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/dws       1458.461s

$ make testacc TEST="./huaweicloud/services/acceptance/dws" TESTARGS="-run TestAccResourceDisasterRecoveryTask_basic"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud/services/acceptance/dws -v -run TestAccResourceDisasterRecoveryTask_basic -timeout 360m -parallel 4
=== RUN   TestAccResourceDisasterRecoveryTask_basic
=== PAUSE TestAccResourceDisasterRecoveryTask_basic
=== CONT  TestAccResourceDisasterRecoveryTask_basic
--- PASS: TestAccResourceDisasterRecoveryTask_basic (2949.17s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud/services/acceptance/dws       2949.202s