Refactor Jenkins directories

Summary:
This change allows more flexibility in Jenkins slave/master distributions:

• each slave has its own directory
• network and SSH key to allow master to connect are determined by "realms"
• slaves can be provisionned on another server than the master

Ref T1422

Test Plan: Respin Jenkins CD master, apsile and elapsi slave containers.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1422

Differential Revision: http://devcentral.nasqueron.org/D1727