Rename "node" to "agent" throughout Jenkins

There is an inconsistent mixture of "slave" and "node" throughout the Jenkins core UI.
For example, viewing the details of a slave shows the title "Slave foo", the breadcrumb "Jenkins > Nodes > foo", and the URL /computer/foo.

As per JENKINS-27268, all instances of "slave" were renamed to "agent", and the changes were merged into the Jenkins 2.0 branch.

Ideally we should take this chance to stop using the word "node" — I think it has much less meaning for regular users, and in general, compared to "agent".

i.e. everywhere where "Node" is at the moment should become "Agent".