Fix problem that unable to get notebook filename on JupyterLab 3.0

[kangwoo]

What this PR does / why we need it:
The notebookapp has been changed from JupyterLab 3.0 to serverapp, so the list of servers is not returned.
So if there is a Jupiter_server module, I changed it to get the list of servers from serverapp.

Which issue(s) this PR fixes
Fixes #551

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

Fixed to return notebook filename on JupyterLab 3.0 

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[k8s-ci-robot]

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[kangwoo]

@googlebot I fixed it.

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

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