ansible: Configure persistent git cache volume for job containers

[martinpitt]

Define a cockpituous-git-cache persistent volume so that the expensive and big node-cache.git (plus the much less expensive bots.git) get re-used between jobs.

Note that we don't want to keep all of $XDG_CACHE_HOME, in particular not the libvirt/ directory. So restrict that to the
cockpit-dev/ subdirectory, which is handled by cockpit's git-utils.sh.

Fixes #611

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I applied this to rhos-01-1, started a local test log, and inspected the resulting volume from the outside:

$ sudo ls -l /var/lib/containers/storage/volumes/cockpituous-git-cache/_data/cockpit-project
total 0
drwxr-xr-x. 1 cockpituous cockpituous 62 Apr  6 08:57 bots.git
drwxr-xr-x. 1 cockpituous cockpituous 62 Apr  6 08:57 node-cache.git

[martinpitt]

I deployed this everywhere. Let's let it run over the weekend.