Fix the API of getting labels from Jenkins

Created by: LinuxSuRen

What type of PR is this?

/kind bug /area devops

What this PR does / why we need it:

Users cannot choose the agent of Jenkins due to the broken API.

Which issue(s) this PR fixes:

Fixes https://github.com/kubesphere/ks-devops/issues/294

I have tested it manually. Please see the screenshot:

See the API implemented in the backend.

Special notes for reviewers:

None

Please use the following image if anyone wants to help me verify it:

surenpi/ks-console:fix-api-labelsdata-dev

Does this PR introduced a user-facing change?

None

This fix is too small. No need to record it to changelog.

Additional documentation, usage docs, etc.:

None