Add jenkins continuous integration server and user.

By default the jenkins server is executed under the user "jenkins". Which can be configured using
users.jenkins.* options. If a different user is requested by changing services.jenkins.user then
none of the users.jenkins options apply.

This patch does not include jenkins slave configuration. Some config options will probably change
when this is implemented.

Aspects like the user and environment are typically identical between slave and master. The service
configs are different. The design is for users.jenkins to cover the shared aspects while
services.jenkins and services.jenkins-slave cover the master and slave specific aspects,
respectively.

Another option would be to place everything under services.jenkins and have a config that selects
master vs slave.
match systemd style and silent curl progress bar during startup check
add platform and license to package metadata.
remove users.jenkins config start on slave config.
Uses standard NixOS user config merging.
Work in progress: The slave config does not actually start the slave agent. This just configures a
jenkins user if required. Bare minimum to enable a nice jenkins SSH slave.
Merge pull request #1831 from coreyoconnor/jenkins

Add jenkins continuous integration server and user.