fix issues in certificate generator

[kevinliu24]

What type of PR is this?

• [ ] bug
• [ ] feature
• [x] enhancement

What problem(s) does this PR solve?

Issue(s) number:

https://github.com/vesoft-inc/nebula-operator/issues/500

Description:

How do you solve it?

Fix the following issues:

1. change certificate validity format to 0h0m0s
2. only start generator if useCertManager is false
3. recreate secret during rotation if secret gets deleted

Special notes for your reviewer, ex. impact of this fix, design document, etc: