tie doc jobs to 'gallium' slave

This migrate various documentation jobs to be run on the gallium slave.
That needs some paths to be updated manually on the server as well as in
puppet (see: https://gerrit.wikimedia.org/r/70998 ).

Jobs updated:
mediawiki-core-doxygen-publish
mediawiki-core-jsduck-publish
mwext-VisualEditor-doc-publish
operations-puppet-doc

Change-Id: I894d9843ddab93d2cc9e027cc11e682bc909a419