Selectively prune /root for ironic-agent ramdisk
Selectively prune /root for ironic-agent ramdisk

Previously all files in /root were ignored when building the
ironic-agent ramdisk. This prevented for example to use the
local-config element to connect to the ramdisk via ssh as root user.

This commit change the exclude rule on /root to only ignore the
/root/.cache directory.

Change-Id: I18d839e8d97636f5f2164ba407f252407d9bc956
Closes-Bug: #1451668

zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,zioproto/diskimage-builder,switch-ch/diskimage-builder