PXE to pass hints to ImageCache on how much space to reclaim

After previous patch PXE triggers cache clean up, if there's not enough
disk space for deployment. However, standard clean up may reclaim too
little space, due to configuration. This patch implements hints to
ImageCache, so that it tries to reclaim exactly required amount
of disk space.

Note that this amount is calculated as total size of images multiplied
by two as an attempt to account for images converting to raw format.

Change-Id: I8f95b7e6334a37e9e6e98278a5b61a9ace7221b5
Closes-Bug: #1316168