zuul: migrate git dir in production to the ssd

The git workspace used by Zuul is on the slow disk. This patch points
Zuul to use the new SSD at /srv/ssd/zuul/git .

Zuul will need to be shutdown and the repository moved from the old
directory to the new one.

Change-Id: Ibbd118f00bd8a76abcc33e203d0928f383b08040
Merge "zuul: migrate git dir in production to the ssd" into production