Push the image cache ttl way up

There is no benefit to ever deleting images from the pxe cache except
when the cache is full. However the cache always consults the image
ttl, so as a short term measure, lets make the default far enough
apart that few use cases will result in it being deleted spuriously.

This patch sets the default image cache ttl to 1 week.

Change-Id: I97fec1336ab2a8aabf8cfd97534bda8d5dea1e85