Adds list of extra plugins available from CI

Display the new plugins available for installation under the
list of the ones currently installed.

Plugins repository is injectable and configurable; at the moment only
the Gerrit CI (gerrit-ci.gerritforge.com) is implemented.

Jenkins CI URL is configured under gerri.config:

[plugin "plugin-manager"]
   jenkinsUrl = https://myjenkins.myserver.com

Change-Id: I7fc5140363700c9edc5c28da460ed8606ed73d78