HHVM opcache in workspace

HHVM maintains an opcode cache which by default is in the user homedir.
Since we can have several jobs running concurrently, have the cache in
the workspace.

Example config:

    Repo {
      Central {
	Path = /var/run/hhvm.hhbc.sq3
      }
    }

Bug: 62278
Change-Id: Id0e176470aaea72170af96b20d5d4db0507ab2c6