cpu-gather: Use actions instead of flags for action argument

cpu-gather: Use actions instead of flags for action argument

This allows for the functionality of cpu-cpuid.py script to be
integrated more naturally in a later patch.

Changes the way this script should be called:
  cpu-gather.py                   -> cpu-gather.py
  cpu-gather.py --gather          -> cpu-gather.py gather
  cpu-gather.py --parse           -> cpu-gather.py parse
  cpu-gather.py --gather --parse  -> cpu-gather.py full

Signed-off-by: Tim Wiederhake <twiederh@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>