restore option namespace in launch arguments

[zw0610]

What this PR does / why we need it:

Restore the launch argument --namespace to allow user to specify namespace to monitor

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #1522

Checklist:

[coveralls]

Pull Request Test Coverage Report for Build 1703120409

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 36.823%

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Totals
Change from base Build 1701643908:	0.0%
Covered Lines:	2272
Relevant Lines:	6170

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💛 - Coveralls

[terrytangyuan]

Please update PR title to be descriptive and fix imports

[terrytangyuan]

Thanks!

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: terrytangyuan, zw0610

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