serverinit: allow dbname 'ui_thumbcache' in high-level config to store thumbnail meta

Change-Id: I0b87c68064691e286734aace9ba794b0dc435523
On GCE, put the UI's thumbnail metadata cache on MySQL too.

Change-Id: Ic3f1238deeb6b569b30e6c3f6b0def2b68f13d93
GCE: put the thumbnail cache on /var so CoreOS doesn't wipe it

Also, move MySQL's data dir so they're both under /var/lib/camlistore

Change-Id: Iffc35efb68a69748ecbe59a6bfc99e9886ae325b