[dds instance]support 0 value of keep_days and support update backup_strategy

[Jason-Zhang9309]

What this PR does / why we need it:

support 0 value of keep_days and support update backup_strategy

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

Special notes for your reviewer:

Release note:

PR Checklist

• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

make testacc TEST='./huaweicloud' TESTARGS='-run TestAccDDSV3Instance_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./huaweicloud -v -run TestAccDDSV3Instance_basic -timeout 360m -parallel 4
=== RUN   TestAccDDSV3Instance_basic
=== PAUSE TestAccDDSV3Instance_basic
=== CONT  TestAccDDSV3Instance_basic
--- PASS: TestAccDDSV3Instance_basic (867.59s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       867.671s