ramdisk: switch from lsb_release to $DISTRO_NAME
ramdisk: switch from lsb_release to $DISTRO_NAME

Instead of executing `lsb_release` to know the current distro being
built, use the $DISTRO_NAME environment variable, already provided by
distribution elements.

Change-Id: I22b67afb481983cc40c198fd408ad5c7f4d68bec

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