Bundle macro + ruby doc using yard

Introduce the `bundle` builder macro which takes a `command` parameter.
The job template '{name}-bundle-{bundlecommand}' lets one easy create a
job for a given bundle command such as `yard`.

We can potentially later on migrate the browsertests jobs to use this
new macro.

Create yard jobs for:
* mediawiki/selenium
* mediawiki/ruby/api

Requires the /rubygems/ directory to be created on the target host which
has been done by https://gerrit.wikimedia.org/r/161004

Resulting output can be seen at:

https://doc.wikimedia.org/rubygems/mediawiki-ruby-api/
https://doc.wikimedia.org/rubygems/mediawiki-selenium/

Change-Id: I10edcf23eec7453c73c2b4e43b2bc6aeb637bafb