Simplify calls to ImageCache in PXE module

This is a simple refactoring to reduce amount of typing when
accessing two kinds of image caches: TFTP and instance images.
It will also make easier to have separate caching settings
for TFTP and instance images caches, if required in the future.

Change-Id: Id3ce989fceb2ee31a93a57d4bdfceaa946b66ff5
Merge "Simplify calls to ImageCache in PXE module"