Always serve plugins folder with or without the `--plugins`
Always serve plugins folder with or without the `--plugins`

without --plugins, serve at `plugins_`, otherwise serve at `plugins`

Change-Id: I91202656ec60501fc826550294e5c5a48479ccca

GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit