Softoff: Add config support for PDR entities

This commit introduces a config file for the soft power off
app. The config file specifies the order of remote PDRs to
be checked during the soft power off process.

Previously, the entity type was hardcoded in the codebase,
limiting scalability. With this change, the softoff app gains
flexibility for future expansions.

TESTED: By running 'obmcutil poweroff' with various remote
entities during boot-up.

Signed-off-by: Sagar Srinivas <sagar.srinivas@ibm.com>
Change-Id: Ib8d2aba6a05fc4e9da4b042fd67890e6282ed784