ibm: inventory-manager: Replace rainier with p10bmc

The IBM rainier machine name is being replaced with the generic p10bmc.

Tested: Built rainier and verified files were installed:
rainier-openbmc-linux-gnueabi/obmc-phosphor-image/1.0-r0/rootfs/usr/share/phosphor-inventory-manager/ibm,everest_associations.json
rainier-openbmc-linux-gnueabi/obmc-phosphor-image/1.0-r0/rootfs/usr/share/phosphor-inventory-manager/ibm,rainier-4u_associations.json
rainier-openbmc-linux-gnueabi/obmc-phosphor-image/1.0-r0/rootfs/usr/share/phosphor-inventory-manager/ibm,rainier-2u_associations.json

Change-Id: I422374d25966e731147eebd1772d5cfc4f62f9d9
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>