Add a test case for DB schema comparison

Add a test case that allows to check if the DB schema obtained
by applying of migration scripts is equal to the one produced
from models definitions on MySQL and PostgreSQL.

Change-Id: I58e2cffd03396223c530d492cd18927c357b90e8
Merge "Add a test case for DB schema comparison"
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