Rebuild master acceptance test cache periodically

[Metallion]

Problem

#96 reuses cache from the master branch to test other branches. This greatly speeds up the CI cycle but a side-effect is that we keep using old images and it's possible that OpenVDC no longer works on the latest version.

Solution

We should rebuild master's cache periodically. I'd say a Jenkins job that builds the master branch with REBUILD set to "true" should be enough. I suggest running this job every weekend. It should keep a copy of the old cache in case things go wrong and we don't immediately have time to fix it.