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
Pin RPC client version min == max

At the moment, ironic's rpcapi module is specifying a target (minimum)
RPC version of 1.0 -- however, none of the methods added after 1.0
indicate in which minor version they were added. Thus, this module
believes all methods are available in any version of the server greater
than 1.0, which is clearly untrue.

This change pins the target version to the current version,
effectively preventing rolling upgrades, which are not supported at the
moment anyway.

Change-Id: I031ccd13d1d140821daf8a0adf5913627064451c
Partial-Bug: #1317300
Merge "Simplify calls to ImageCache in PXE module"