overload connectionCheckUrl for use as base of plugin url

[onejli]

implicitly use server hosting update site as connectionCheckUrl

[jenkinsadmin]

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[onejli]

Just noticed that this PR is a little stale. @kohsuke are you the owner of this repo?

[kohsuke]

Connection check is meant to test the reachability of the network, by hitting a server that's far more reliable than Jenkins infra. That's why it points to Google.

I believe your intent is to change where the artifacts get downloaded from, and if so, it should be added as a field to MavenRepository and MavenArtifact should get the value from there.

[kohsuke]

And I forgot to mention, but my apologies to the delay.

[daniel-beck]

@kohsuke A case can be made that http://www.msftncsi.com/ncsi.txt is a better default choice for users in countries with censored internet, see JENKINS-23654.

[onejli]

@kohsuke @daniel-beck I'd proposed these changes because I'm operating an internal-only update site within a secure environment without access to the outside internet. In cases like this, my thought had been that using the update site itself for the connection check url would be a relatively sane server-side check. This is because at the time that I made the changes, Jenkins was relying on the client's browser to fetch the update-center.json, but was retrieving the plugins themselves via server-side operations (which introduces the possibility of being affected by different network ACLs for server and client).