new extension point: PluginInstallJobFactory

This pull request adds a new ExtensionPoint to the UpdateCenter configuration. 
It allows new implementations of how a plugin should be installed, 
e.g.
* download via URL from a mirror (Default Implementation)
* from a Maven Repository

The new first implementation of this extension point (https://github.com/imod/repository-updatesite) will allow administrators to install plugins behind a firewall from a internal/private maven repo mirror of http://repo.jenkins-ci.org/public/
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