pkg-map: respect --missing-ok also on missing pkg-map file
pkg-map: respect --missing-ok also on missing pkg-map file

When there is no pkg-map file for the specified element, list the passed
packages if --missing-ok is specified.

This mirrors what is being done already later, when a pkg-map file
exists but it does not have the mapping for the requested package.

Change-Id: I50287f8e6a5e4335deb3de2252075b8bfdd53ce5

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